the 6600 GT would actually beat the 6800 standard card in some cases.
Especially if you get the XFX Geforce 6600 GT Extreme Gaming Edition for about £150
I read a bit of Custom PC and they tested laods of 6600GT's and prooved on this one to be the fastest of the lot. Especially since they could overclock it way more above XFX's standard overclock to squeeze even more FPS from games. Plus its AGP
On there test system they got about 56fps from farcry running at 1024x768 (no advanced effects)
at 1,028x1,024 2x AA and 2x AF they could squeeze 46.6FPS out of it. Not bad I don't think.
If you want to run at the highest settings liek res's of 1,600 x 1,200 with 4x AA and 8x AF, this card isn't for you (only high res monitors can reach this anyway) which gives you 23.5FPS in farcry, which is below proper playable standards of 30FPS.
Half life 2 on 1,02 x 1,024 2x AA 2X AF = 55FPS, but 60.7FPS in standard 1024x768
Doom 3 at the same spec = 43.7FPS, but 65.4FPS in standard 1024x768
All in all, a very good card and will have no probs running new stuff for a while as long as you don't want to shove the settings up too high
It is way worth your money. People say not to get standard cards, or SE cards as they are toned down from ones like PRO's, but even this is lower than a GT and prooves to have everything squeezed out of it in most cards cases.