we have a 6 year old computer it has a 2400 AMD Athlon processor 768 meg of RAM shedloads of Hard Drive (about 360gig including external) and currently an ATI/Radeon 9200 graphics cars (put in earlier this year)
we need a new card because we want to run "Company of Heroes" - supported cards - Geforce 3 or better, or Radeon 9500 series or better. We'd like it to be future proof for this kind of game for a couple of years and want to spend less than £100, ideally less than £50
the 9800XT was the dog's danglies in it's day. Good find! It will require an additional molex power connector from your psu. Also you should check out the size of your psu in case it's below the min specs for the card.
Graphics chips are made by either Nvidia or ATI. There are dozens of graphics card manufacturers (Gainward, Leadtek, XFX, MSI, Sapphire, Sparkle, 3DFX, Powercolor etc etc etc). Many follow the Nvidia or ATI reference design very closely, and the cards will perform almost identically. For these shop on price, software bundle, warranty etc.
Some manufacturers in an attempt to get a higher price for their cards overclock the gpu, fit faster memory, fit more memory, overclock the memory, or install bigger / better / quieter cooling solutions. Go for these options if you want to squeeze the last ounce of performance from your card, if you want to boast about it, and if you have few budgetary constraints.
If you really want to save money have a look at these refurbished cards from Retek. Your £50 budget could be a practical proposition then. The X800XT strikes me as particularly yummy at that price point.
(beware a couple of the cards are PCI-express, not AGP)