I'd look for at least a GTS250 (or a 9800GTX+ which is about the same thing). A 240 may be better than a 7900GS, but not by all that much.
Personally, I would save up for at least a 4XX series, or even consider going ATI as the 5770 and any of the 58XX series cards are very nice.
But yes, like TD mentioned, you need to find out what you can about your power supply. If it can't handle the load, you can damage the rest of your system (and fry a brand new video card).. or in a best case scenario it won't work properly if at all.