You will have absolutely no issues using the AGP version of this card with your system.
'GT' badging indicates a supercharged version of a chipset, followed by 'Ultra' for the nitrous-powered version. 'LE' is nVidia's coding for a little bit slower, but a LOT cheaper.
However, for not much more money you can have the 6800LE, the cut-down version of nVidia's flagship graphics chipset (6200 is entry-level, 6600 is mid-range, 6800 top-end). The 6600GT is blistering for the money (<£130), but a few more pennies will give your modern apps/games the latest graphics options, and run them a little faster too.
Having said that, I run a 6600GT (AGP) in one of my systems, and a 6800GT (AGP) in the other, and, at this time, the framerate difference is there, but it's beyond what the human eye can detect (i.e. anything over about 30-35 fps).
It's a good card.
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