Please, please don't get any 7600GS with DDR2 memory, or any card with DDR2 memory. Please. And a 7600 GS with DDR3 memory is basically a GT (possibly also in price, so why not get a full GT if that's the case). Even a Galaxy 7300GT with 1.4 (AGP) or 1.2 (PCI-E; discrimination... evil; as if AGP can't hold it - they are just trying to kill the platform) ns memory chips and a Zalman cooler outperforms 7600GS.
I would advise against a series 6 card because the newer DirectX and games pop up, the more the newer series stand out. For example, in older games, my old 5900XT would own 6600 big time. But in new stuff, it would get outperformed by a measly Radeon 9550 (which I consider a thrice less powerful card). It's true that generation 6 still rules in games, but the same fate as that of my 5900XT will become their share soon, I think.
If you really want a 6800, I think you should get a GS or an Ultra. Ultra is more powerful, but GS is newer and probably contains newer components, like from series 7 cards as vendors boast. GT has more threads and streams and whatnot than GS, but GS scores more in 3DMark. All in all, I would probably go for a series 7 card anyway. I've just bought a 7600GT for $178 in my currency and I think I'm happy with it. I haven't had the time to play a newer game yet, but Warcraft III TFT seems to have improved a lot graphically, compared even to 5900XT on max in-game settings and max or close quality in the driver.
And we're talking about an old game not designed with that card in mind because it simply wasn't around yet. Maybe I'm seeing things though.
I still have to try NWN2.
Depending on how much you want to spend on the card, I suggest taking a look at Radeon x1950pro - the strongest AGP card at the moment (16000 in 3DMark03 compared to 13000 for 7600GT, which is already more powerful than 7800GS, which is the "strongest" AGP one from nVidia unless there's something I don't know about - maybe the GS2 thing?). All in all, that card was able to break my preference for nVidia but I just didn't want to spend so much money it cost here (mine +66% and the difference in efficiency was +25%, while I was already overpaying compared to Galaxy 7300GT 1.4 ns Zalman edition for -32% cost and -22% efficiency, so...
).
And once again, DDR3 is your keyword.