Another thing, someone told me that 4Gb DIMMs run slower than 2Gb ones, is that true?
Lol no. Running 4 2GB Dimms causes slightly slower access times than running 2 4GB Dimms. In reality the difference doesn't matter, but you want 8GB, and better to leave slots open our be able to reuse them.
Running 2 2GB dimms would be faster than one 4GB dimm though, as they are meant to run in pairs