So, here's the story: I can play SCII at ~20 fps at the lowest settings on my onboard Radeon 2100. No crashes, works as well as one would expect. I bought a GTX 260, and could get ~40 fps on all ultra. Only played maybe 5 matches on it, but it worked perfectly every time. I was informed that I could have gotten a slightly better card, the GTX 460, for only $10 more, so, I return the 260 and order the 460. 
 
Received the 460 in the mail today and popped it in. Updated to the latest drivers. Seemed to be working as well as I thought (40-60) due to my X2 5000+ being a bottleneck. That's a problem I'm taking care of tomorrow by ordering the Phenom II X2 555 @ 3.2GHz (On a side note, is this a decent replacement? Will SCII run at 60?) Anyway, every single match I play freezes up, accompanied by artifacts covering the screen for 10-15 seconds, then returns to normal. This happens every 3-5 minutes. ( This is at medium settings. On ultra, the PC has to be hard reset). Looks similar to this mockup I made, only the squares are flickering. (Probably normal artifacting, but I'm trying to be as thorough as possible here!) 

I would check other specs compared to the GTX 260 you returned. Also, check reviews on newegg and other hardware sites. Their could be problems. I remember when I bought an nVidia 7600 card and a month later upgraded to a 7800. There was a lot of frame rate problems.

I'm playing at ultra on a GTX 275 at 1680x1050 and getting min FPS of around 45, average 55.
 
It seems those fermis are terrible cards.

You might want to use this when you play SC2.

I have the1Gb version of the GTX460 and it's amazing (for the price, especially). I bought Starcraft 2 yesterday and have not had any errors like that, and no overheating. Under 50 seems like a very good temperature.