I'm confused about whether you want hard drive space, RAM, or both.

Anyhow, if you buy a bigger hard drive you might want to check on your motherboard's BIOS to see if it supports larger drives. If it can't, maybe it can be upgraded (you'd check with the BIOS manufacturer, then download and install the update). If it can't even be upgraded, you can always buy a PCI card to handle the larger drive, taking the place of the onboard IDE. It's just something to check; I've seen it happen a couple times.

Good luck.