there are 2 main graphics card compaines at the moment, ATI and Nvidia. They both do ranges of cards for £50 to about £300. The mid range card you would probably want would be a ATI 9600. There are 3 versions of this card the standard (9600), PRO (9600 PRO) and XT (9600 XT). The performace of the XT and PRO is better than the standard but they are more expensive (about £100 for std and £150 for pro/xt).
The Nvidia equivilent is the FX 5700 (2 versions standard and Ultra) is fairly similar in terms of performance and price so it doesnt really matter which one you choose. If you want a cheap basic card the ATI 9200 and FX 5200 are only about £40 - £50 and can play pretty much any game you want but at not such high quality settings as with the higher end cards and it will not be as 'future proof' (i.e. you will have to get a new card in a year or so to keep up).
hope this helps, graphics cards are rather confusing at the moment .
ADDED:
Sorry, i just read the title of your post again. As long as you have an AGP slot in your board it should be compatible with any modern graphics card.
the agp slot looks like this...
ADDED AGAIN: Just rembered somthing else. If you PC is older than 1998 - 99 then you may have the old syle of AGP, this runs at a higher voltage and can kill a new GFX card. Check the motherboard manual (if you have one) that its running at 1.5v not 3.3v.
