Phase 1 - Get together a solid foundation that I can expand on in future phases. This should get me a working system with minimal cash outlay, while ensuring that the most important/expensive parts won't need to be replaced in future phases.
Phase 2 - Watercooling and overclocking. Being a watercooling noob, I'm after a simple-to-install watercooling system that will allow me to push the Phase 1 system a little. It should also allow me to extend it later on. (And remember - my girlfriend has to approve, so it can't look like it was built by a noob!) This should set me up for running multiple hot GPU's in future phases.
Phase 3 - 2-way SLI. This is where I will add another GeForce GTX480 to the mix, along with a couple of extra screens, for a 3x1 portrait setup using nVidia 3D surround. I'll try to find a couple of 22" 1680x1050 screens, identical to the Samsung I have right now. Supporting 5040x3150 should give me a new performance target to aim for.
Phase 4 - New, dual-processor motherboard, with a 2nd Core i7-930, in prep for Quad-SLI. Hopefully, EVGA will have released this one by then. 8 overclocked i7-930 cores ought to give me as good, or better, performance than 6 i7-980X cores, for less money.
Phase 5 - Stage 2 watercooling and overclocking. Tentatively, this will involve a pump upgrade, (or second pump,) another radiator and a full-cover motherboard block. (This will likely have to wait for someone to make a full-cover block for the new mobo installed in Phase 4.) Time to see how far that new dual-pro motherboard will take me!
Phase 6 - Quad-SLI! I know that the 4th GPU probably won't gain me much in performance over the 3rd, but what the hell - I'm here aren't I? I'm already having to drain my loop - might as well finish the job! Hopefully, nVidia will have improved their quad-GPU scaling with a magical driver release, by this point. If not, the 4th GPU might end up relegated to PhysX. It also seems distinctly possible that games will start offloading more stuff to DirectCompute and so take advantage of more GPU power. This phase will only go forward when I can't get playable framerates on new games out of the previous phases.
Phase 7 - Not sure about this, yet. Possibilities include CPU and RAM upgrades. Stereoscopic 3D is also a possibility, but that would involve 120Hz monitors, which don't seem to be available with anything approaching small bezels at the moment. Depends on what I'm playing at this point, I guess.