3840x1024 with a triplehead2go digital edition. its what prompted me to go for the 8800GT 512, since i felt the 7800GT 256 would still be okay since i reduced the resolution to being 1920x480, though, i didnt like using it at that res for all that long for many games.
so, after increasing the resolution to the max supported, i took more of a performance hit than i cared for, and proceeded to upgrade as such. and saw more limitations than there should have been 'why didnt the playability improve, even though i just spent $250 on a new gpu???'
checking 3dmark scores, they were just as they shouldve been too (using futuremarks orb system), so no problems there, it was at 1280x1024 so its a low res, just as youre saying... i did find out though, that by OCing my X2 3800+ to 2.6 with the 7800GT, that certain scenes in 3dmark06 saw increases in fps in low fps areas (parts of return to proxycon for example), i started finding out more and more though after that realization
edit: no, the 7800GT wasnt AS fast as the 8800GT at very high resolutions, but there was still enough of an impact for me to exclaim 'what the heck!?' and wonder just what was going on. things were improved, but not nearly as much as they shouldve been (such as in oblivion, which was a game i had been wanting to play on the th2g since reading thg article on the th2g analog edition) especially after reading how close the 8800GT 512 was to the 8800GTX, and resulted in the remaining upgrades being well worthwhile, where i really wouldnt have believed so otherwise (512mb is still largely enough though at that res, even if the 768mb of a GTX and such would have been more ideal still).
although, i did read something awhile back that someone mentioned in another forum, something about the difference in cpu architectures that they had read, which allowed the core architecture to maintain a higher fps in some intensive game situations than what i had been seeing, which had me wondering too, that was before i had upgraded anything though, so i only vaguely wondered, but didnt care all that much either, since what i had was fast enough at the time, and had been for the nearly 3 years that i had it.