128-bit vs. 256-bit is a bit of a red herring. Notice that the 7600GT's RAM is clocked a lot higher than the 6800GS's and GT's, so while it has less bandwidth, it's not exactly half as much. Future games like Oblivion will probably favor shader power more than bandwidth, in which case a 7600GT should play better than a 6800GS or GT. While HDR does require more bandwidth b/c it's working with bigger numbers, it also tends to require more shader calculations, and so a 7600GT basically maximizes its lead over a 6800GS/GT with HDR; the same for gaming without AA. Only when you enable AA (probably 4xAA or higher) will bandwidth impinge on shader power, bringing the 7600GT closer to the 6800GS and GT, but it's still faster in the newer games that most sites benchmark with.
I'd get a 6800GS over a 6800GT, and a 7600GT over a 6800GS. Just shoot for what's a) in your budget and b) fast enough for you at your preferred resolution and AA setting. Stock, a 6800GS should be a little slower than a GT, but just buy one of the OCed models (e.g., BFG's or eVGA's) and you're good to go. An added bonus of the newer cards, the 6800GS and 7600GT, is that they draw less power, and so dump less heat into your case and can make do with quieter coolers.
It's pretty easy to see how a 7600GT compares with a 6800GS at stock speeds
here. 16xAF is on for all the benchmarks, so the differences are no AA, 4xAA, HDR w/o AA, and HDR w/ 4xAA. You can see that the 7600 is fastest without AA, and the 6800GS closes the gap with AA--but is still slower. It's easy to figure out bang/buck using those numbers, too. Just divide the 7600's relative score by the 6800GS's, and that's the max percentage more you can pay for a 7600 and have it be at least as good a card for your money. Just keep in mind those scores aren't average framerates, just relative scores normalized to a default value of 100 given to the 6600GT's average score in the first graph (so those aren't average framerates, just relative points).
But looking at that chart, and reading a bunch of reviews that compare a 7600GT with a 6800GS, you can see that it's not a heck of a lot faster than a 6800GT, so it won't be a huge upgrade. Still, if you can sell your 6800 for a good price, then the upgrade might be worthwhile.
BTW, there's an X1800GTO in that review, too, if you're considering it.