I may have a quick and easy solution for you. No upgrade to PSU and a completely capable GPU that will work just fine with your current PSU and only one 6-pin connector.
I've had zero issue with it. I also upgraded from a 7900GS. 430W Antec Tru-Power Trio(I know it's not big a beefy, but it runs without a hitch, and when I do upgrade my PSU, the card will fly more than it already does).
edit: I botched the image link..here it is on Newegg:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150316XFX 9800GT. I picked up a 22 inch widescreen monitor at the same time I ordered the GPU, runs everything I can throw at it. Runs a tad warm so just crank up the fan a bit and you're good to go. It is also relatively short compared to most of the EVGA cards. I was originally looking at the EVGA 9800 series, until I realized they were about 10.5 inches long on average. I forget exactly how long this one is..just longer than a PNY 7900GS by about .5 inches.
I'm no computer genius, although I've spent plenty of time tinkering with my current system and systems past. This card runs plenty stable and should be fine in your system. Tildeath's advice should be heeded though. This card also came with a Y adadpter, but the XFX installation card does -NOT- recommend using it, even as an adapter to a single 6-pin PCI-E connector.
