the MSI overclocks well, but the DFI overclocks the best. all the world records for Athlon 64 overclocking has been done on the DFI board.
I've seen an FX-55 at over 4GHZ on one of these boards.
anyway, I'm not here to tell you "get the DFI"
I can recommend it to you, but it's up to you whether to get it.
and definately don't go Radeon X series. the 6600 GT will outperform the X800 GT and X600 (it will double the X600's speed)
bu I would try and get a 6800 plain, and softmod it to get 16 pipes.