I'm finally getting rid of my single core and not a second too soon and I need some input if my new build is going to perform alright. 
I'm not going for a top of the line rig, but something that can play most modern games especially RTS.
 
Here's my build 
- Asus M4A77TD motherboard
- XFX 5770 graphics card
- Athlon II X3 440 CPU
- Kingston HyperX 2x2 GB Dual Channel (PC3-10600 @ 1333mhz)
- Corsair 550W PSU
 
I'll use my old case and harddrive since it's full tower and the harddrive is brand new.
Thanks!

I'm not too familiar with AMD processors, but I'd definately get a better graphics card if you want to play modern RTS games on max settings.

Seems fine to me, if you've got a budget that can stretch a bit, I'd recommend moving up to a quad core rather than the tri core since you're aiming at playing RTS games which generally require a fast processor over a beefed up graphics card (not that a good graphics card isn't necessary).
 
My recommendation, change the X3 440 to a X4 635 which is a $23 increase and replace the 5770 with this recertified gtx 260 unless you really want DX11 or need a cooler system overall since the 260 has a slight edge over the 5770.

a 5770 is fine for what games you are playing it doesnt seem necessary to spend to spend more. AMD has quad cores that are not much more than that tri-core like Commandercup said so that would be a good upgrade to do. 

I'm not sure you would be able to play Starcraft 2 with that graphics card, let alone more graphic intensive RTS games. I might be wrong though.

Ok I swapped out the cpu with a Phenom II x4 945 and the motherboard to one that has 1x16 pci-e but runs with 8x if crossfire is enabled. So if the 5770 isn't performing I'll just add another later on - I've read it's pretty close to a 5870 in performance when you do that. Hopefully that's a little better. I also swapped out the PSU to a more potent one if another gpu is needed.  I'm really stretching my budget now :)

"I'm not sure you would be able to play Starcraft 2 with that graphics card, let alone more graphic intensive RTS games. I might be wrong though. "

StarCraft isn't a very graphic intensive game. Strangely enough, it doesn't even utilize quad cores. Of course, there are other computers that uses them. SC and RTS in general, from what I heard, are more CPU centric games than GPU. Having a good CPU is good enough, just don't anything fancy with them.

I would go for the Nvidia GTX 460, which has been getting rave reviews and really is the best bang for your buck these days. If you add another one of those later you would have a very good gaming rig. Two GTX 460's in SLI are straight up beating two ATI 5870's in crossfire. You say that you want to be able to play most modern games, and if you want to play them with high settings, go for the 460 and add another one later. 
 
From hardocp.com :

 GeForce GTX 460 1GB SLI real world gaming performance is superior to even AMD’s fastest high-end single GPU video cards in a CrossFireX configuration. And do keep in mind this is real gaming, not benchmarks.