I recommend a lot of GPUs, but I need a price range. @OP: Post your computer specs. Power Supply and CPU mainly.
My power supply is around 500W yeah. But I'm talking in British pounds. Not sure about $-£ but £1=€1.12 ATM
So the limit is ~$160 US dollars... Although I would consider waiting for the lower end Kepler line to be released...the GTX 460 is probably your best bet right now.
http://www.newegg.com/Special/ShellShocker.aspx?cm_sp=ShellShocker-_-14-125-396-_-05212012_3 I know it's US, but it's still a great price.