Perfect timing! I just recently got a new video card and have been meaning
to post here with my experience.
I ended up getting the Sapphire HD4650. So far, it's working fine.
It's actually in my daughter's PC. She's been beating on it, playing
all the games that matter to her, and no problems. It's definitely a
big improvement over the integrated graphics, but we haven't tried
to measure it.
I have to say, I'm still nervous about the power supply requirement.
(The box agrees with the 400W power supply recommendation.)
If you play games that are extremely graphics intensive for a long
period of time, it still might be an issue. But we haven't seen any
issues so far.
Oh, and I did spend a bit of time trying to locate a larger power supply
that would work in the slim case, but no luck.