Juan,
A couple things you will want to know before buying additional hardrive space and RAM
1. SATA Or IDE ( I would imagine you have sata inputs if you running a new pc)
2. Speed, make sure the drive is 7200rpm
3. Cache Size, 16mb is good if you are doing a lot of read/write
4. RAM, I recall you have a Pentium D? I belive PD's take DDR2 at a maximun of 800hmz. Check the guide for you motherboard, if you put in 4x500mb sticks. the motherboard many not be able to run in duel channel, which basically doubles the memory bandwidth.
5. RAM, if possible, make sure the RAM is of the same brand and spec. You could have 4 different sticks of DDR800, but they may run at different timings.
MikeyDB mentioned a SCSI drive, that would be consider the "high end" of things, if you need absolute performance, look into a Western Digital Raptor, which run at 10,000 RPM, same speed as a SCSI, but it has Serial ATA connections.