As far as rotational speed, 7200rpm. Won't get quicker than that due to faster rotational speeds causing more noise and heat, two things that are generally avoided in laptop design if possible. (Hell, there aren't even very many drives > 7200rpm for desktops...). You gain a little speed on larger drives due to areal density, but in real-world terms, not going to be noticeable over the 100GB 7200rom you have now.
The other option is a solid state hard drive (flash memory drive), although these are wickedly expensive even for small capacity such as 32GB at the moment... hardly worth it yer IMHO.