OuterSquare, I went to Mwave and got prices for the Maxtor DX740:
20G = $77 = $3.85/G
40G = $96 = $2.40/G
60G = $125 = $2.08/G
80G = $168 = $2.10/G
On a strict $/G basis it looks like the 60G is the deal, just as BrunoPuntzJones said. Practically anything in the 40-80G range is pretty competitive, and the decision would be based more on needs than $$.