A new video card you say?!
Okay If I am not mistaken, you have about $150 (U.S. equivalent) to spend. You've got a few options available to you, especially since your system has an AGP upgrade path, which is a shock considering most systems with integrated graphics do NOT give you the option of doing so. That's a step in the right direction there. With a PCI video card, the fastest performance you will get with anything is about equivalent to 1500 3DMarks in 3DMark2001. And if you do your homework, the fastest machines paired with an Athlon 64 FX model 51 with a GeForce FX 5950 Ultra will rack up about 20,000. But, that's another subject entirely.
My advice to you would be to shop around a bit. If you do any gaming besides with emulation, and are not going to be playing anything on the cutting edge but still want good pixel and vertex shader support, and will not be using anisotropic filtering or FSAA, an nVIDIA GeForce 4 Ti4200 or even a Ti4600 is a great option. I have an ASUS V8420S myself, and they're great cards (they come clocked out of the box at 261MHz core and 554MHz memory!

) and perform well in all DirectX 8.1 apps. If you're looking for something a little more future-proof, Your best bet would be either a GeForce FX 5600 or an ATi Radeon 9600 Pro. And out of those two, the ATi card will blow the hell out of the 5600 non-ultra.
All of these cards should be less than 120 UK pounds or $150. Hope I've been of help!