New cards run on PCI Express slots which your board will not have and to upgrade will be very costly.
As you'll be running AGP 4x or 8x I'd go for a 7600GT if you can find an AGP version or a slightly older second hand card like an X800 or X850 ATI (XT or XT PE model) or indeed a 1950XT if you have a bit more cash.
A 6800ultra or higher card will run Doom 3 easily at high resolutions. Just aim to spend £100 and get AGP....oh and don't buy it from PC World! Try
www.overclockers.co.uk for the best deals. (No I don't work there)
You could always clock up your chip and ram a bit depending on what you have got. This will shift the information around a bit quicker and means the card will get the info faster (if you have 1gb of DDR2700 which is likely upgrade to 2gb).
Just some thoughts
Dan