You want to stick with PC-3200 DDR2. If your machine takes DDR2, the slots are different than plain old DDR RAM, so you cannot simply throw in a stick of DDR RAM. If you buy something slower than the current speed, both RAM modules would use the slower speed. For example, if you have the PC-3200(400MHz) chip and a PC-2700(333MHz) chip, both would run at 333 MHz.

Depending on the motherboard, sometimes you can use sticks of different capacities. Some computers require you to install RAM modules in pairs of two equal capacity chips. Some motherboards really don't care. In your case, it looks like another stick of 512MB would be fine.

The best place to look is the manual. Try and hunt around for a motherboard manual, it will tell you any specific rules the motherboard has for RAM instalation. Also, newegg is normally much cheaper than Best Buy, and I have never had a bad experience with them.