DO NOT GET THE GEFORCE 4 EQUIVALENT!
What is your budget?
I'd suggest the Radeon 9500 which is about £100.00 I think.
It's made by 4 or 5 manufacturers under license (sapphire Hercules etc.) and if you shop around you'll be able to get a deal on it. It's £100-£170 cheaper than the Radeon 9700 with only a sight decrease in performance, and it's directx9 compatible so is as future proof as you can get atm.Get the 128MB not the 64MB.
Overclockers.co.uk have a good deal on it.