I'd go for the Gigabyte DS4 mobo, Conroe E6600, 2x1Gb Geil PC6400 Low Latency RAM, Tagan 580w modular PSU, Antec P180, WD Raptor HDD(s).
The x1900xt will be a damn good card, but if you have the money then by all means go for the x1950xt ;)
I'd stay away from the Seasonic/P180 combo as the...