It comes down to what your buddy is charging for it. Don't confuse the GF6600GT with the FX5950U. Talking new games, their is no comparison whatsoever. If the FX5950 were half the price of a new GF6600GT, fine, but otherwise a GF6600GT, or even radeon 9800 Pro would be much better for the future. IE, don't pay too much for that FX5950U if it is future DX9 titles you hope to play. Older games it will fly, OGL probably be very good too. But in Direct X 9 titles, the weakness of the FX series are out in the open, and it isn't going to get better either. NV's drivers team is putting their time into the 6xxx series now, so FX optimizations will be not be a top priority for them. Not to mention game developers are probably going to spend les time trying to get the FX series cards to play their games well. They (GF FX's) struggle with the standard DX9 codepath, and need their own specific path, or to just default to DX8.
All Geforce FX's, including the once mighty FX5950, do struggle in some new DX 9 games, WHEN forced to run the DX9 codepath. Take HL2 for example, the FX5950U defaults to the DX8.1 codepath like a Radeon 8500 series card would run. It does well in DX8.1, but forget seeing the nice water reflections that only come running it in the DX9 codepath. When forced to run DX9, the FX5950 is unplayable. 3Dmark05 also exploits the FX series DX9 weaknesses. I'd say, yes it will game pretty well, but for future Dircet X games, it will play like a beefed up GF4Ti series card(DX 8 card) as you won't be able to run the DX9 codepath.
<A HREF="http://www.digit-life.com/articles2/digest3d/1204/itogi-video-hl1-wxp-1280.html" target="_new"> HL2 Performance Chart</A>
<A HREF="http://www.anandtech.com/video/showdoc.aspx?i=2281&p=3" target="_new">DX9 visual's and GF FX performance Hit</A>
<A HREF="http://www.digital-daily.com/video/3dmark-2005pro/index04.htm" target="_new">3DMark05 too much for GF FX series, NV optimizing drivers for FX series less</A>
<A HREF="http://service.futuremark.com/compare?2k3=3400555" target="_new"> My</A>
<A HREF="http://service.futuremark.com/compare?2k1=8268935" target="_new">Gamer</A>