How do those cards compare to the Radeon 9800Pro? A big speed increase? I haven't bought a graphics card in a while.
Yes, pretty big. You'll also be able to have anti-aliasing and anisotropic filtering enabled without any performance loss. Pretty sure the 9800 was a SM 2.0 card as well so you'll be up to SM 3.0 with these. In order words, you'll be getting better looking effects, soft shadows, things that you weren't seeing on SM 2.0.
I'm inclined to agree with Halo. I don't see the point in going beyond a 7600 if you're sticking with an AGP card. I don't know ( and perhaps you don't either ) when you might want to upgrade the motherboard, cpu, etc, so I would probably play safe and pick up some kind of 7600 for a bit less money. It's not likely to be pushed to it's limits any time soon.
Something like this, perhaps? 256 meg of DDR3, and it's £125. The PCI-E model would probably be cheaper, but that's often the case with older bus hardware.
http://www.novatech.co.uk/novatech/specpage.html?NOV-76GTA( Not particularly suggesting Novatech are the cheapest, although they're usually fairly good on price, they're just local to me. )