That is the 1 billion dollar question everyone wants an answer for!
By the time you wait for 45nm CPU's and DX10 to smooth out, there will be 40nm CPU's and DX11 on the horizon. It's entirely a personal choice when you decide to "jump in".
dude DDR3 sux! honest to got, look at this http://www.xbitlabs.com/articles/memory/display/ddr3.ht... . and 45nm CPUs are gay 65 are just fine! get an E6850 and 2Gb of DDR2 ram at PC-8000 speeds and a little water cooling here and there and youve got a 4Ghz stable comp which will last atleast 3 years, u just need a GFX card that wont choke like a 2900( optimised for DX10 not 9 like 8800)