I can't afford to buy a new system, or do a full upgrade, so I basically need the cheapest, best option, to replace my ageing AGP X800pro.

Any suggestions?

X1950 Pro for about £100 would be a good improvement

Anything for half that price would be fine

Whats the rest of your PC, specifically your CPU, RAM and PSU.

3.0GHZ P4, 1gb ram not sure about the PSU, 500w I believe.

I am not looking for (huge) performance gains, just better compatibility with new(er) titles.

TBH an x1950 will give you a nice performance boost, but you might want to consider dropping AGP - a PCI-e version will be faster (although not for the reason you'd think) and come in at £30 cheaper.

I can't afford to upgrade my motherboard aswell, which would be the best bet.

...then the x1950 Pro is your best bet, but the P4 WILL hold it back, although you will see a performance boost.