I recently upgraded to a Radeon 6870 1GB, However its not running nearly as well as it should. Battlefield 3 struggles to stay above 35 frames most of the time on High settings. Fallout 3 maxed out dips into the 20s when Im looking at few buildings or theres alot of people. Metro 2033 also struggles to stay above 35 frames on High settings, not to mention on I have AA turned off. I have to think its my PSU which is a Thermaltake 320 Watt, which is way under the recommended 500 watts for the 6870. Is it definitely my PSU or something else?

GPU: 6870 1GB

CPU: Core2 Quad Q6700 @2.66GHz

Memory: 4.0 GB

OS: Windows 7 64 bit

Im playing at 1920X1080 resolution.

I had very similar issues with my Radeon HD 6990 about a year ago, and changing the PSU fixed it. Curiously enough I changed to a PSU with inferior wattage (went from 900W to 850W), but something else about it was better. Still not sure what. I was advised to get it, and it worked.

I won't say I'm 100% sure that'll fix your issue there, because your CPU may be seriously bottlenecking your graphics card. Still, you definitely need more wattage to get the performance you seem to be aiming for.

EDIT: Also, this should be in the PC forums.

Three hundred and WHAT? jeez... Yeah, you need more than that..

@MordeaniisChaos said:

Three hundred and WHAT? jeez... Yeah, you need more than that..

You should also look at something called the 12V rail. I'm not too keen on what a rail actually is, I just know some video cards also have that in their specs. Something like "500W power supply with at least 25A on 12V rail". Usually new power supplies have something like 30+A on the 12V rail. My Corsair 750W has 62A for example.

@myketuna said:

@MordeaniisChaos said:

Three hundred and WHAT? jeez... Yeah, you need more than that..

You should also look at something called the 12V rail. I'm not too keen on what a rail actually is, I just know some video cards also have that in their specs. Something like "500W power supply with at least 25A on 12V rail". Usually new power supplies have something like 30+A on the 12V rail. My Corsair 750W has 62A for example.

So that might be why I am getting way better performance on my current 850W PSU than I was experiencing with my old 900W one. I'll look into this, thanks.

It could be your CPU honestly. I stuck the 7850 I'm currently using into a core 2 quad machine (though it was 2.4GHz) and a lot of games would get iffy framerates and the GPU was only running at around 50%. 
 
Fast forward to today and I have an i5, those games run perfectly fine. GPU at 100% and everything.
 
So it may be a CPU bottleneck.
 
You NEED a new power supply, though. I'm amazed that thing hasn't gone up in smoke. 
 
 
Also the 12 Volt rail is the power rail your video card uses. The higher than amperage of that rail, the more power you can deliver to your video card(s). If you have a power hungry card (like a 6990) it will need more amps to deliver the watts it needs. A 25A 12V rail would give you 300W of power to your video card. I believe an overclocked 6990 may actually use more than that, but 300W is plenty for a single card of almost any other variety.
 
 
edit: For a single 6870 a 500w power supply should do just fine, assuming you don't have a crazy amount of peripherals.

@Slaegar: No way is that CPU causing such shitty performance on Fallout 3. I used to max that game out on a 3Ghz Core2Duo.

OP make sure power saving is turned off in the ATi control panel or dynamic performance scaling, whatever it's called. I wouldn't jump to the power supply yet because usually computers lose power when the demands of the PSU are exceeded.

320W may, or may not be enough depending on the strength of the 12V rails, that 500W estimate by manufacturers is BS as some cheaper 500w PSUs have terrible power distribution and might not be enough either.

I'd get a Corsair 400+w PSU if it comes down to it, they have really strong 12V rails and long warranties. (unless you are going for dual video cards, you don't need anymore than 400w, hell even a good 620w will power dual video cards with room to spare)

@Sooty: When I built my computer last year, I went with a 650w Corsair HX modular PSU to power two 6950's. It has been more than enough...one of the reasons I went with that particular unit was the 12v rail rating, 52A dedicated single rail. It was a little pricey, but it's a beast and was totally worth it. I don't see why I would have needed more than 650w.

@BigBoss1911: I think even my current phone has a 350w PSU. Definitely time for an upgrade dude!

@Snail: it was the efficiency rating, probably. The wattage refers to the maximum it'll draw, not its true output. eg a cheap/bad psu might draw 900W, but lose 150W in the AC-to-DC conversion.

Not surprisingy, these PSUs also generate more heat, which tends to shorten their liife.

I doubt a lack of wattage can cause a decrease in framerate. If the PSU isn't capable of delivering what the system is sucking, it'll just melt, or cause the system to shut down. It shouldn't cause decreased performance, I'm pretty sure at least, I've never heard of it before.

@MAGZine:

The rating on the power supplies is supposed to address the amount of power they are capable of delivering, not the amount of power the suck out of they wall socket before the conversion loss. The 900 watter might have been a stupid cheapo that listed 500 of the 900 watts on the 3 volt rail, where it is not needed. while the 850 might have been proper and actually able to deliver a decent amount on the 12 volt rails.

Still cant believe a lack of wattage would result in an fps drop as opposed to something blowing up or crashing though. I've actually had a power supply die on me cause I used a bit too thirsty components on it. It ran fine for a week, and not affect my frame rate during that period.

I have a 650 and sometimes wonder if it's enough power for my GTX560Ti. But yes, you need far more power. You should go for a 650+ PSU if you can.

Just get a good quality one that is above 550 and you´ll be fine.

It should not be your CPU that is bottlenecking. I have a Q6600 with a Radeon 5770 and I can play those 3 games at 60 fps with medium/high settings. Though I do run at 1680x1050. Just try lowering your resolution first? But honestly, 350 watts is a bare minimum on todays machines. You really should upgrade that either way.

I'm surprise a 320 watt PSU even has the right connectors for the card. Yeah you'll definitely want to get a better PSU then that, you can cause trouble with your computer