X1950Pro is probably the best AGP card at the moment, although I have read some speculation about an AGP X1950XT being released.
As wjrt says, make sure your psu is up to the job.(check out the detailed power requirements of the card) I'd guess you'll need a 500w from a reputable manufacturer. Don't be tempted by any of the alleged 500w £25 bargains; be prepared to part with £60 or more.
If you tend to upgrade your computer in parts then now may be a good time to switch to PCI-express. You can replace your motherboard for about £55 click here so when the time comes to upgrade your processor, there is no need to replace your graphics card again.
If your next upgrade is likely to be a complete overhaul then go for the AGP card.
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