im sorry but theres NO WAY a 8600 gt 's core GPU can pull off 1 gig of video ram. theres a test online with 256 mb and 512 mb 8600 GTS. the 256 is actually at some point slightly better or slightly worse than the 512mb. so just get a 8600 gts 256 mb SC for like 130$ and itz uber good.
probably different clocks, and more ram doesn't really play a huge factor unless your gaming at higher resolutions, then 1gb of gddr3 could be quite handy.
It seems good since you tried it on older games, and youre comparing it to only a 6800, which is basically an older version of the 8500gt. But run a dx10 game and youll be disappointed. If you had a more poweful card to compare it to, your opinion would change. I too didnt game too much, but after i got cod4 i was amazed so i bought an 8800gt and its great.