If I'm keeping my old mobo as a spare setup - and want to upgrade the RAM on it and already have 2 x 512 sticks of corsair PC3200 (400mhz - 200mhz), can I just grab 2 x 1gb sticks (any brand) and chuck them into the remaining slots (PC3200 of course).
In most cases you will be OK with two 512 and two 1024 chips. (assuming they have the exact same CL rating, matching manufactures is a good idea) If running in Dual Channel mode you would run them in:
512 | 1024 |512 |1024
I highly suggest you check your motherboard manual to make sure this configuration is supported on your motherboard. Also some older systems may have a memory cap. I had a P2 400 Mhz with a memory cap of 512 MBs.