with 120mm fans in 5v configuration you'll definately want to run them on both sides of the rad, as they won't have very much static pressure at all - as BB2k said.
I'm sure you'll find one big heatercore and two silent 120mm's will work perfectly fine, but if not there is nothing to stop you from simply adding another heatercore in the loop
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2x P3 1100's at 1400, Abit VP6, 2x Corsair 256mb PC150 sticks, 20gb 'cuda ATA-III, 2x 40gb 'cuda ATA-IV in raid 0. 20" Trinitron. No fans
2x 2400+ at 2288mhz (16.0 x 143), Iwill MPX2, 2x Kingmax PC-3200 256mb sticks, 4x 20gb 60gxp in Raid 5 on a Promise SX6000. Asus Ti4200 320/630. Cooled by Water
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