Thinking of adding another gig of RAM...

[BlackRider]

do I have to worry about matching it to my current stuff?

I have this:
CORSAIR XMS 1GB (2 x 512MB) 184-Pin DDR SDRAM DDR 400 (PC 3200) Dual Channel Kit

Which is actually more now than what I paid for it last year. :S

[XMEN Iceman]

Just make sure the speed matches. I take it you have an additional 2 memory slots?

[XMEN Gambit]

Don't have to match brand or size. But you'll need to get pretty close in the speed department, and of course if you're running dual-channel now you'll need to continue with dual-channel. Whenever you mix memory, the whole system will usually match the slowest, safest settings available.

[XMEN Ashaman DTM]

And in those mixed-memory situations, if you try to push the memory, you'll have plenty of stability problems.