The xfx xxx version has a higher clock, so in theory it will be slightly faster. However, EVGA has a step up program meaning that if any new cards come out with a month (I think it's 1 month), then you can upgrade to the new card free of charge.
Yes. A larger frame buffer will get you more fps and a better performance at higher resolutions. Bother the evga and xfx 8600gts are $169. you should get a HIS 3850 for the same price as the 8600. it will perform better and have better image quality. http://www.newegg.com/Product/Product.aspx?Item=N82E16814161211
The HD3850 is MUCH better than the 8600, but memory is also important, so get the 512MB version. As for "Superclocked" or "XXX" edition graphics cards, don't bother, you can just overclock them yourself.