Well I put my eVGA 6600GT AGP in my machine yesterday and so far I have no complaints. I don't have City of Heroes or Splinter Cell so I can't really compare results, but playing World of Warcraft at 1280x1024 with all the features turned up, I'm getting anywhere from 40-75 FPS, whereas with my 5700LE I was getting 15-25. Performance in Everquest 2 is also much, much better, although I'm not sure how to find out FPS in EQ2 so don't have a number
So far I'm happy with the 6600GT, we'll have to see how it does over time.