Seagate 7200.10 320GB has one of the lowest costs per gigabyte, and it uses only two platters, which is advantageous for reliability, noise, heat, and power consumption. I used one.
I needed something bigger, so now I have the 400GB because it was a better deal than the 500GB, but I didn't really want to get it, as both use 3 platters, so the 400 is the less optimized of the series.
If you really want big the 750 is the one with the best density, and the price now is not bad.