I've already got 1 gig of ram. If I buy two sticks @1 gig each, can I add them?
Or do I need 2,4,8, etc. Will 3 work or will it only count 2?
Also, can I mix DDR2 and DDR3 RAM?
I have a Dell Dimension E521
According to Dell's product page on the Dimension E521, you can upgrade up to a maximum of 4GB.
The service manual shows you have four DIMM slots, so you can install an additional pair of 1GB sticks. I doubt your system has four 256MB modules in it, so you shouldn't have to remove anything. You probably currently have either one 1GB module, or two 512MB modules.
To answer the second part of your question, no, you cannot mix DDR2 and DDR3. Your computer only supports DDR2, so make sure you purchase the right type. According to Wikipedia:
This has nothing to do with the operating system. You will need to read the motherboard manual in order to discover what combinations may be supported.