ok it really doesn't matter that much wich cpu a fast core 2 duo could do the trick,
(for a single gpu like your 280)
perhaps an i5 750 quad core will suit you best,or go ftw with an i7 920,
i have only seen cpu limits when you install more than 2 gpus (nvidia or ati)
and found out that actual core duos and quads make a little botteneck but for getting
the best gpu scalability go for the i7 920 its around 280 $ ,plus x58 motherboard
around 200-250 $ and some ddr3 memory for 120 $
price gets some benefit you can run sli of crossfire on this motherboard.
still the best cpu & motherboard would be an i5 750 for 200 $ and a p55 motherboard from 100 $ (single pcie 2.0) to 280 $ for dual pcie 2.0 (notice that
p55 now admits both sli and cf on some motherboards like the x58 motherboard
and to finalize ddr3 dual channel is a bit cheaper than triple channel for about 80 $ you can get 4gb ddr3.