Read times are around 8ms for a 7200 drive, 4-5ms for a 10K. Dunno the read times for the 15K
Hard drives fail folks.
Everyone needs a second as a spare. So 2 smaller drives are better than one big one.
I have 2 in my system, a 75G Raptor as well as a 35G Raptor, had em both for a few years now.
Even when you have unrecoverable issues with one drive you can often transfer data off it before it becomes unusable either from hardware failure or a corrupt OS.
Install your Operating System on both drives, it will only boot from the primary and ignore the OS on the second drive.
If you ever have a problem you just swap the wires and it boots off the second drive keeping you and your data safe.
Even if you are one of those who feels the need to re-format regularly you can clear one drive to the other(documents, pics, dowloaded programs), do the format, and swap data back fast without all that downloading...looking for disks...
Whatever drive you have now, get a small 10K RPM drive(or faster), run your programs off that as your primary, any drive is fine for storage...anything but games really.