RPM is 'Revolutions Per Minute', and with hard-drives it's a measure of the hard-drive's speed. The hard disk has to physically spin to access data stored on it, and if it can spin at 7200RPM instead of 5200RPM, that'll bring an obvious speed increase
Practically, you'll see a speed up in the time to copy files, and the system will generally feel a bit more responsive.
However, it usually causes a slight decrease in battery life, somewhere in the ballpark region of 5-15% on the average laptop.
Overall, I'd go with the faster drive, though. Unless you're really depending on every last minute of battery life, it'd be the better choice for your custom laptop. ... Especially if you're wanting a speed machine
