Help understanding CPU Usage, RAM, Disk Usage...

[Bezo]

I recently invested in BFD(CPU hungry drum program), at the same time upgrading to Tiger and PT7. So CPU usage is now an issue for me. But I need to understand what upgrades will effect performance.

Currently, I have on 1GB of RAM. Before I max it out at 2GB, I need to know what that will actually improve.

CPU Usage, in a session I'm working on, is topping out around 50%. What exactly is this messuring? CPU processing power, RAM usage...? And how does the hardware buffer size and CPU Usage Settings play into this?

And there are 2 other usages monitored in Pro Tools. Disk Usage and another that escapes me at the moment. What are these actually measuring, and what upgrades improve there performance? And why would these show no activity while CPU Usage is at 50%?

Not sure if I'm asking the right questions. My understanding is that 2 primary things effect a CPU's performance. Processing power and RAM. Waht I'm trying to understand is their relation to the 3 Usage Meters in Pro Tools sessions.