I've been running the following system for a while:
C2D E7300 (currently clocked up to ~2.8ghz)
ASUS P5Q SE Plus motherboard
3 gig Corsair XMS2 @ PC26400 (2 x 1 gig modules + 2 x 512 mb, dual channel)
BFG GeForce 7950GT OC 256mb
Antec Trio! 430 watt PSU (3 rails @ 18A, 36A combined total on +12v)
I'm hoping to get a new GPU in the next month or so, as the old girl is starting to show her age in newer games. My maximum budget is roughly $150 (I'd very much like to keep it closer to 100 bucks though). From what I've gathered, my top contenders are the Radeon 4850 and GeForce GTS250. I doubt highly that a 4870 would be feasible at this point, due simply to my power supply - tempting though its now low-low price may be. The GTX260 is a hair out of my price range, but I might consider it as well.
It looks like it's going to come down to a battle between power consumption and CPU bottleneck when attempting to shoehorn a new GPU into the system.
So, I charge you, dear forum readers - what, based on your experience, is going to give me the best bang for my buck in this price range? Usually, I'd just do this myself, but school has been taking my attention away from the important things in life (read: sweet, sweet graphics processors).