If your system utilises dual channel memory architecture, and it almost certainly does, it's normally necessary to have the same size RAM stick in each slot, or it will operate in single channel mode with reduced performance. If I remember correctly, a few boards can split the difference and run partially dual channel, although that might be dual channel boards with 3 RAM slots.
How much memory a 32 bit OS has access to will vary between systems, and can be up to 3.25GB or occasionally more. So it could be worth getting 4GB and selling the existing pair of DIMMS.
If you fit DIMMs with differing timings, it will probably operate, but run both at the slower timing. If they were the same size but with different timings, it could be expected to drop from dual channel back to single channel too.
Edited by Platypus, 11 December 2010 - 07:20 AM.