Depending on how long you want it to last you, you probably want to go with a full 4 GB.
The 9600 GSO your looking at (and unfortunately the only kind they sell anymore) has a 128 bit memory interface. If you could find a 9600 GSO with 384 MB RAM or 768 MB RAM and a 192 bit interface, that would be the card you saw in those benchmarks. The new dumbed-down version doesn't even compare since the memory bandwidth is reduced by 33%. The 8800/9800 GT would be a good choice, but they were out of stock. I originally recommended the 4670 because it doesn't require a 6 pin from the psu.
If you have a six pin or two, it doesn't really matter which graphics card you choose (in terms of compatibility) If you want to check, just open up your pc (when its unplugged) and look at the cables the come out of the psu; if on looks like the one in the link below, your good; if not, with the exception of upgrading your pc (or using an adapter if you have extra 4-pin molex connectors) the 4670 is the best you can get.
http://www.techaddicts.net/reviews/st60f/st60f.13.jpg
This is a good reference when finding out which card is better (again however, the 9600 GSO listed here is the old 192 bit edition)
http://www.tomshardware.com/reviews/best-graphics-card,2404-7.html
You could also look at the performance charts and find the games you plan on playing
http://www.tomshardware.com/charts/graphics-cards,1.html