nvidia 6600gt or ATI x800gto (the latter just released) Both extremely capable cards, available in PCI-e and AGP flavours, prices around £100 now and likely to be falling a bit further as new models released towards the end of the year.
the difference between agp and pci express is the bandwith basicly pci express is faster ,but you will not be able to get one of these unless you motherboard supports it ? thats why i was asking if it is a old pc then i would say it would be agp, or is it new ?
If you want to replace all 3 components LOW do some good value bundles. this ones an MSI Nvidia N-Force 4 PCI-express mobo, Athlon 64 3500+ processor and X800GTO graphics card which has 256mb GDDR3 memory, all for £308.
Problem then is you'd probably also need 512-1024 mb new RAM to do it justice, and in all likelihood a new power supply too(decent make, 450W with the required connectors)which should set you back a further £100 or more.
Still, you'd then have a pretty zippy machine which will run most of todays games well, which is what you were after in the first place.
I would strongly advise against getting a ati x800 graphics card as does not support open gl.I installed one two weeks ago and found that I could not play games that use open gl such as Area 51 Doom 3 Pariah Painkiller Halo just to name a few of my games that will not play.Ati are supposedly going to issue a patch to sort this out sometime.Until they do give them a wide berth.I have been visiting game support sites but have still not been able to resolve this problem