Attempting to make this unbiased as possible (though I am probly the biggest oss/apple fanboy on these boards)
I'm currently running a 12" powerbook with osx, it's my main rig, and the most useful IMO, we will leave it at that, as this is a windows themed thread, and my opions are made clear arround here.
My primary windows machine is a dual 2.2ghz amd mobile XP machine, 2gb ram, 2gb ram, lots of hdd (including a raid array) and a tnt2 video card (don't ask.) it's primary use is video editing.
Vista will happen when it happens, If you keep waiting you will never upgrade. But I have a feeling that harware requirements will keep increasing as trusted computing/HD content becomes mainstream. I've read online talks of requireing a HDCP link from video card to monitor, and as of now there are very few/no machines that support that. The TPM module is a whole nother story, and scares me to think about it.
DirectX 10 looks promising, but again, market acceptance won't happen overnight. You seem like one of thsoe people on the bleeding edge, and thats cool, but game developers will code for what most people have. And that will still be dx9 hardware for awhile. And at that point it gives you some buffer to buy new hardware once the specs finalize. (think about when PCIe came out and nvidia and ati were still releasing new agp cards)
If I was you i'd up your ram, maybe get a decent graphics card you can afford, and keep the cpu. SMP is still very over-rated for anything but pro content creation.