Unless you are mad into gaming you will notice no difference whatsoever and even if you are into gaming I would be extremely dubious. Save £80 and buy an external 80Gb Freecom HD from PCW...it will save you many times over.
external hard drive I can only imagine was recommended for back up. but the reason I say AMD is cos yes the reliability is good, also you can overclock AMD CPUs and Pentium 4s dont even run at the speed they say on the box.
...no good having a fancy processor and graphics cards if you lose all the info on your computer...external HDs are the way forward. You can also dump programmes that are infrequently used on them rather than using your main drive.