SCSI gives the best transfer rate

But.. that's expensive, hot, and loud and I doubt you want to do that.
Or, you could get a 10,000rpm WD Raptor - but I think those are stupid and I'll tell you why. A Raptor spins faster to move data faster - that's great and all, but it sacrifices size and it creates a lot more heat than a normal hard drive. You can usually buy a 250gb PATA (Parallel ATA - old style) hard drive for the same price as an SATA 74GB Raptor.
However, the larger the hard drive, the more dense the data. So, in theory, a 400GB hard drive with two 200GB platters would have to travel less distance (thus spin slower) to transfer the same speed as a 100GB hard drive with two 50GB platters.
So.. after this.. here's what I'd recommend: Buy the hard drive with best price to storage ratio regardless of interface. The transfer speeds among hard drives are not going to get you better frame rates in video games. Hell.. the transfer rates between 7200rpm PATA and SATA hard drives isn't exactly that great. Fast hard drives will get you faster boot times, unzipping, cd-burning, or game-loading..
But, if you're like me, you might not want to trust all your data to a single drive. So, in that case, keep two hard drives in your computer and make sure your important data is on both of them.
Hope that helped.. now go browse the Hotdeals forum to get a good deal on a hard drive.