It says this:
So. Can I replace just one of the modules with a 2048 chipset, or do I have to put one in each? Can I have a 2048 in one module and leave a 512MB one in the other? If not, will it explode if I try?
You can have either one or both slots occupied. The RAM size of each slot can be different.
If you currently have two 512MB sticks in each slot and replace one of them with a 2GB stick, you will end up with 2.5GB of RAM. If you currently have one 1GB stick in one slot with the other empty, and you put a new 2GB in the empty slot, you will have 3GB of RAM.
If you have RAM of different frequencies (e.g. a 4200Mbps/533MHz and a 3200Mbps/200MHz), the RAM will work at the slower of the two speeds.
A good place to check out memory is www.newegg.com . It will tell you what RAM will work with your specific motherboard. Note that some motherboards rated for a max of 4200 will run 6400 RAM sticks at the 4200 speed, and some motherboards won’t support the 6400 sticks at all. So make sure you buy sticks that are compatible with your specific motherboard.
A great place to find deals on RAM is www.slickdeals.net
You could also try this to determine which RAM to buy:
Er… What he said. 
Just a nitpick, but from the info given, the memory must operate at 4200 Mbps, and 3200 Mbps memory will not work and faster memory (PC5300 or whatever) is still run at 4200.
My understanding is that the 4200 is the rated maximum RAM speed that the motherboard can support, which means slower DDR2 RAM will still run but at a speed slower than the rated maximum. Faster RAM may or may not work depending on the motherboard.
ETA: To be more precise, the slower RAM will force the motherboard system bus to work at the slower RAM speed, but it will work.
Many thanks, all.