I've finally had the chance to try some games....
Doom 3 - Game plays quite well at 1280x1024, high detail. Have tweeked the doom config file for better performance. Game generally performs very well, much more effortlessly than before. It can slow down a bit when the screen gets very busy though, probably down to as low as 20 fps, but at a guess it seems to maintain at least 40 fps most of the time. Clearly my CPU probably holds me back a bit here. Curiously, the first time I tried it did not accept the res and I was playing it at 800x600. Thinking it didnt look too good and didnt run as well as expected, when I eventually discovered this and finally switched modes, it ran faster than at 800x600
Painkiller - Tried it first at 1600x1200, with high detail, and generally it ran quite well, but there was a period where it slowed right down. Switched to 1280x1024, and the game seems much faster and more responsive now, barely slowing down even on the busiest sections (and trust me, that is very, very, very busy). I'd guess this was running at 50-60 FPS on average.
Worms 3D - OK, not really a particularly good test by all accounts, but it seems to run well enough at 1600x1200. Probably even more so at 1280.
To all intents and purposes, it is very difficult to tell the difference in image quality between 1280x1024 and 1600x1200, so using the lower resolution out of the two is a great balance between speed and quality with this card, even on my ruthlessly revealing 22" CRT. Anti-aliasing may improve some of the graphics even more, but is it needed? Not really IMHO, and I'd rather have the fast frame rates.
It is however, clearly better than my old TI-4200 even ignoring the fact it is using high resolutions and detail levels - the 2D windows picture is far better than it was before. As Gary has complained with about his 9800, this card is fairly noisy, and has raised the noise level of the PC substantially..... I guess this is the price of wanting high performance. Having said that, the noise is not irritating, and is relatively unobtrusive.
Coming later will be impressions of Far Cry, Serious Sam 2, UT2004 and others.