I just recently got a WD Raptor @ 10k RPM SATA and I absolutely love it...now I am not a huge person on benchmarking..so I haven't got any technical specs for ya...
However, what I am able to say is my games load WAY faster...installing Windows XP Home was nothing short of amazing in the time it took..
So in all over "peppiness" of my system...I would say seeing is believing...and I am definitely a believer of my SATA Raptor.
So, depending on what your planning on upgrading from, I would definitely say it is worth it