^^^
Exactly what he said.
i915/925 mobos are, in a way, transitional. They will never support a dual core cpu, like the i945/955 does, if you should ever decide to try a dual core, down the road.
ref: First sentences of
THIS ARTICLE
The only real advantage of i915/925 over i945/955, is you can buy i915 now, at a reasonable price, whereas you'll need to wait a bit for i945/955 to mature & come down in price.
IMO, I really don't see i915/925 currently bringing a great deal more to the table, over that of your P5P800, except for the capability to run an extreme high-end PCIe16 gfx card, & having HDD NCQ + native RAID capability in the ICH6R SB.
BTW - My current highest performing machine is a P5P800/640/G.Skill combo, and I'm perfectly happy to wait a few months for i945/955 + dual core cpu's to hit the streets, before I add another cruncher to my fleet.
JMO's
Strat