I made this same decision a couple months ago. It was between the 6800GS or the 7800GT CO, same thing, $100 price difference(relatively) at the time. I was building a brand new rig that would be well rounded(not necessarily an ultimate gaming machine or nothing).
I bought the 6800GS, and I regret it. I needed a video card, and I needed one "now" at that time. I wish I would've done 1 of 2 things, A: bought the 7800GT(a much better performer overall, 1/3 the price premium, but also 1/3 the performance gain, roughly..runs newer games better), B: buy some cheap video card to get me by for the next six months and then upgrade to something that would be a quantum leap above the 7800GT for possibly the same price(IE: 1900XT...with more cards coming out, there will certainly be price cuts). Yes, I can upgrade 6months from now anyways, but the fact is, I blew $200 on a midrange card, I would've taken a less hit if I went with say.. a 6600GT(can be had for around $100) or maybe even something cheaper that was pci-e.
This is just my opinion though, and that is how I would've done things differently, if given a second chance.
To reiterate why I think the 7800GT is a better buy(questionably at this moment though...) over the 6800GS is that the price per performance is worth it a lone...but also it will have longer gaming life, once you spend $200+, mind as well spend that extra buck for something that will last. My 6800GS hesitates to play NFSU2 at 1600X1200, and that's an old game! I haven't had a chance to play any of these newer games, I'm planning to get into quake4 and CS:S later this year, when I have time.