The concept of "matched pairs" belongs to the era of 72 pin EDO RAM roughly 486/Pentium I processors. Some early PIIs also have the same memory.........this one (PII/266) does, for example. [Before any "purists" start flaming me, there are exceptions where there is fixed memory on the MoBo]

Later PIIs and after either use SIMMS or DIMMS which are independent and can be added as you wish, provided that the MoBo/BIOS will support that particular size of strip. I have come across problems where someone has bought a 512Mb strip that won't work, or is recognised as a 256Mb because of the MoBo/BIOS limitations.

An exception is the RAMBUS RIMMS (PC800) which also needs to be installed in matched pairs. This memory is pretty rare though, and belongs to the era of the early P4s.

Yes, you can get away with mixing memory types, sometimes. I do not recommend it though, and would say that memory should be of the same speed (PC2100 etc) and clock latency (CL2/2.5/3) for the most stable installations.

You could add a 512Mb strip to your machine to give you 1024Mb total memory. The difference is size between DIMM strips does not matter. In theory you should have the largest one in the lowest numbered slot (0 or 1) which is usually closest to the processor.

Unless you are into very heavy duty graphics type apps, I do not think that you will see much improvement beyond the 512Mb you already have. You could REPLACE it with PC3200, or look at upgrading your graphics card, if you want better performance.

Hope that helps