GPU upgrade for playing Diablo 3 on PC with intel Q6600 cpu

I would like to play diablo 3 on my computer with minimal problems. I currently play on the lowest settings even lowest resolution and still only get about 15-20fps (sometimes unplayable at 15 or lower). I know my comp is old, 4-5 years,and needs upgrades but i would only like to upgrade the PSU and GPU at this time as i will prob be getting a new computer or try to upgrade other stuff later on. I plan on only playing Diablo 3 and possibly WoW. Wow is playable but the diablo 3 beta isnt so great. specs are;

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01...

Intel Q6600 CPU 2.4ghz
4GB RAM
Nvidia 8500GT 512MB
PSU 300W

So what is the best GPU for me to get after upgrading the PSU. These two will be the most graphically demanding games on my pc but i will not likely be playing them at the same time. I do prefer Nvidia but simply because that all i ever had. my budget is around $150 more or less for Vid card and $80 prob for psu. give or take 10-15 dollars on each.

have thought about gtx 460 or gtx 550Ti but i dont know if its all compatible or what would be best for my pc at this time.