It's a tricky situation. There are so many hard drives out right now that have great performance compared to older drives.
$55 is really good for that disk, but performance wise there's probably other large disks that have similar numbers.
gun to my head, I'd buy the $55 velociraptor and put the saved money into another component, unless i didn't have a budget and was building a sick compute.r.
http://hardforum.com/showthread.php?t=1400507
Somebody posted in this thread w\ screen shots of 300gb vs 150gb vs 74gb velociraptors. It shows around 100-125MB/sec for the 150gb version.