I have the X800 Pro AGP version on my P4 gaming rig.
I like it a lot and Half-Life 2 looks fabulous

As for which interface to get, well since you do not have your new box yet, perhaps look at motherboards that support PCIe (PCI Express).
I have seen some very nice boards out there like the Asus P5GD2 Premium and also an SLI rig from Asus as well, based on the nForce 4 SLI chipset for Athlon 64/FX cpu's.