X800P should have the same board size as the XT, and probably the same as the AIW XT. The AGP XL is (much) longer because it has to fit an extra component, the PCIe->AGP bridge chip, as the XL uses a PCIe-native GPU.
It'd be a shame to discount the 6800GT just because of HD placement. Surely you can move the HD up or down a slot to make room? I don't think that'll make much difference in terms of heat. $300 is too much to pay for an X800P considering a 6800GT should be about the same price but be faster. I don't think it draws enough extra power to make a big difference.
Yeah, your CPU will be holding you back, but OC'ed to 2.4GHz (with a corresponding boost in FSB), it should be decent. You could always get the video card and upgrade to an Athlon 64 and an AGP motherboard later, if you feel your CPU is holding you back. The most future-proof upgrade (both in terms of future CPUs you could just swap in and future video cards) would be to get a socket 939 Athlon 64 and a PCIe motherboard, but that'll cost you more.
Here's a power supply calculator if you want to see how much of a PSU you'll need for your (current and future) components.
Way to enter the fray with a troll, Rollo. Not everyone keeps up with hardware as we do.
Edit: And since when has a CPU bottlenecked a frickin' HD? The HD is *always* the slowest component in a case, and
his X15 has a seek/access time half that of any current IDE drive short of the Raptors, including
your new 250JB. He doesn't need to upgrade or ditch everything in his system just to play games better. I think he should first try a new card with his OC'ed Northwood (just move his HD over), then maybe consider the added expense (and time) of a whole new system.
Edit the Second: Well, if the noise bothers you, just ditch the HD. Speed is nice and all, but not at the expense of comfort. I had a Quantum Fireball that whined at all times and sounded like marbles dropping whenever it did something. It was a huge relief to retire it, and since then I've focused on noise (read: silence) first, speed second. If you want comparable speed at much lower noise levels, get a WD Raptor 74. If you don't do anything that requires SCSI-like seek times, I'd just get a Hitachi, Samsung, or Seagate 160GB+ HD on sale. Like I said, $40-50 after rebates and taxes isn't bad for a quiet, reasonably fast drive to replace all three of your current ones. You can keep one of the old ones and occasionally plug it in to back up important data; otherwise, I'd keep it in the case but disconnected (it'll probably be noisier than a new HD).
If you're interested in a quieter system, give SilentPCReview.com a read.