Is it better to get two 512MB sticks or one 1GB stick in order to get one gig? Also, is it better to choose a higher frequency when the PC requires, for example, 533MHz PC4200 DDR2 SDRAM?
It's true: Two smaller RAM sticks will perform better than one larger stick. Because the PC can access both sticks in parallel, so your computer can (theoretically) have access to twice the RAMage as it could if you had only the single RAM stick.
Vendors like to use every possible RAM slot in order to take advantage of this effect. It wasn't uncommon to see a server with eight RAM sockets, each featuring a 128MB RAM chip, for a total of 1GB of RAM.
Realistically, if you aren't running a web server off your computer or dealing with some application where every ounce of computing power counts, you aren't going to notice much of a difference with, say, a 2x512MB vs. 1x1GB RAM configuration. However single 1GB stick is about 10 to 15 percent cheaper than two 512MB sticks.