Western Digital Caviar Black 500GB 3.5" SATA 6.0Gb/s Internal Hard Drive -Bare Drive
Western Digital Caviar Black 640GB 3.5" SATA 6.0Gb/s Internal Hard Drive -Bare Drive
Western Digital Caviar Black 750GB 3.5" SATA 6.0Gb/s Internal Hard Drive -Bare Drive
Western Digital Caviar Black 1TB 3.5" SATA 6.0Gb/s Internal Hard Drive -Bare Drive
Generally speaking within a given generation of hard drives the larger drives will perform better. This is because they'll be able to cram more data into each cylinder, and that will reduce the need to move the head as much.
But the performance of the drives in a given generation will still be fairly close. So in "dollars per performance" terms the smallest drives will be the best bargain because they're the cheapest by a wider margin than the difference in performance. But that's not a very good way to differentiate if you want speed, IMHO.
Best Price/Performance is a Samsung Spinpoint F3 1TB. You can find them for $55 often enough and they match WD Caviar Blacks in performance.
If you must have a Western Digital, make sure you get one with 500GB/platter. I'm pretty sure the 1TB drive is like that. I dunno about the others. Reliability--the fewer platters the better. If the 500GB drive is single platter, that's probably the way to go. But price/GB is higher.