well im using a HD5570 on a 250w psu. ur pc is old but it might work. it plays most games at medium/high. im currently playing through bad company 2 at medium settings at 1280x1024.
You can do this to get by if you need a card now and can't wait.
Other wise save that 200 and put it towards a new computer, you can make a pretty nice computer for 400 dollars, I just built my friend one for 360 with a bunch of combo deals and mail in rebates.