The X700 is a good choice for a mid-range AGP card. It has 8 pixel pipes, and if you get the 256MB version, you will get reasonable performance on medium settings. Here's a guideline of game settings for a midrange card :
Resolution : up to 1280 x 1024
Filtering : generally up to 8x to 16x, higher filtering doesn't generally cause much of a performance hit in my experience.
Anti-aliasing : generally none, but occasionally up to 2x depending on the
game. AA is a performance hog on any card, but a game like FEAR will most likely be unplayable on an X700 unless it is switched off.
v-sync : not needed unless you go LCD. Again this is a performance hog and should only be used on high end cards and LCD monitors.
Texture detail : medium to high, will require some experimentation.
Shadow detail : medium
Soft shadows (FEAR) : off, huge performance hog
Particles : medium
These settings will give you a very solid gaming experience in most games on a mid-range card. Settings obviously vary from game to game, and we're all used to experimenting with them. I generally leave some of the more nonsense features like soft shadowing off because you can barely notice their effect, and they can be a huge performance hog. Don't be afraid to crank up the resolution up to 1280x1024, just don't go to 1600x1200. The Doom3 engine does not benefit from AA at all, and generally any aliasing effects are greatly reduced simply by opting for a higher resolution. That said, aliasing effects in all games are reduced naturally at a higher resolution, but with Doom3 its much more pronounced.