after the power supply wat next
Well, if you get a new powersupply, say a good brand name of 450w+, then you can start looking at cards. I wouldn't recommend anything over an ATI 5770, as the rest of your system will hold it back. Unless of course, you are planning on upgrading part by part over time, then a 5850 could be ideal, but a hell of a lot cheaper by the time you might be able to actually use its power.
What is your budget? What do you want it to play?