Want to upgrade RAM, don't know what to get I'm not sure if 32/64 bit makes a difference but I'm assuming it doesn't. It's just memory.

Dell XPS 400, recent but fairly old. I have two 512mb cards for a WHOPPING TOTAL of 1GB of memory! I'd like to at least add 1-3GB and if my visual analysis is correct, I have two extra slots. More than 4GB would be useless because this is a 32bit.

Is there just one size of ram or what? Can I just plug it in and the computer will register it? I also know that there are ddr2, ddr3.

Help. Complete newcomer to actually doing something but a little knowledgeable.