With a budget that large you have a lot of options open to you.
Ultimately the video card is what will hold you back; this review gives a good perspective:
AM2 is an intermediate step between 939 and AM3, its lifespan will not be as long as Core 2 duo - in my opinion.
Availability of processors, support, and motherboards is much greater than the AM2 however.
But with a large budget I think Conroe is a good choice
I am looking into buying the Asus P5B-deluxe
And an Core 2 Duo E6400 - once they become available and the tytan Newegg sells them.
Also, apparently this RAM will overclock to DDR2 1000+ with CAS4 timings... (not 100% positive on that)
that would be well under a grand, then you could spend money on some outrageous video card like a 7950GX2, or wait around for a DX10