BleepingComputer.com: Dual Channel Memory

Hi,
I have four slots for memory for my PC. 2X 512MB RAM is preinstalled leaving 2 more slots. Does that mean that I can have 2X 512MB RAM and 2X 1GB = 3GB RAM and yet get the most proformance out of it. Or is is that I have to get them all to be identical (4X 512MB or replace 2X512MB with 4X1GB?)?
I'm not nessesarily upgrading to that amount, but it could be revelent infomation for the future.
Thanks!

They do not HAVE to be installed in pairs, but it is better if you do so.

Ex. 512MB x2 + 1024MB x2

In order to run in dual channel your modules have to have the exact same specs, they can be of different manufacturers but you will need to populate the expansion slot with the same amounts of RAM. Slot 1 and 3 has 512MB modules, if you put a 1GB module in slot 2 you will want to put a 1GB module in slot 4.

There are those that will tell you that you don't have to have "matched" pairs to run in dual channel, but if you do manage to get it working with mismatched modules the performance may not be much more appreciable than running in single channel, if you plan on using this computer on anything that is demanding then you will start to notice problems. You get what you pay for.

But how about in different speeds? For example: if the pair of 512 Mb were 400 Mhz, while 1 GB pair were 600Mhz. Will that reduce proformance or the system will only use 400Mhz of the 600?