Granted, but it does the largest bang-for-the-buck in most systems with 512 MB (really even 1 GB nowadays) or under. As for the other ways, they tend to reduce disk clutter and make the OS utilize the most out of the HDD; the amount of data given 'immediate access' priority is RAM-dependent.
Anyway, to the OP...5 GB is complete overkill unless you do some very specific, very heavy resource-dependent tasks (and no, rendering video while playing Crysis doesn't count). Don't take this the wrong way, but if you don't know how much RAM your system needs, I'd bet the house that you don't need more than 2 GB at the absolute most (probably in the 1 to 1.5 GB range). What Series Pentium D do you have? The 9xx's were great, but the 8xx's were iffy. Personally, I'd pull out all but 2 GB of RAM, Defrag your HDD at least two times (preferably three) right after each other, run a Disk Cleanup, then schedule a full disk check (the kind that needs to have the system be restarted in order to do) and see where that gets you.