The faster the rpm, the shorter the 'seek' time. In other words the faster your Hard Disk spins the quicker your PC will be able to get the information it needs.
That is only the first part of the equation.
Secondly, how is you HDD connected to your motherboard?
ATA(IDE) 133 is the fastest normally, but a lot of PCs still use ATA 100. SCSI is faster, but expensive. Serial ATA (SATA) is just coming out and is roughly the same speed as ATA 133 at the moment. It will get a lot faster, but you need a special HDD and a SATA controller on your MoBo.
Thirdly, does the HDD have a Cache? Some do - mine has 8MB - this also lowers seek time.
Finally, your swapfile/paging file size also contributes once the program/file has been found.