It depends, if you are going to run it in dual channel then the modules will have to have the exact same specs. If you are going to run in dual channel you will find that there are "kits' sold by manufacturers that generally will be less expensive than purchasing them separately. You can use modules of different manufacturers, but as I mentioned, they will have to have the exact same specs, there will always be some differences between modules produced by different manufacturers and if you are doing something demanding like three D imaging then you may find that the matched pairs that come in kits are better.
If you are not going to be running dual channel then it doesn't matter if the modules are from the same manufacturer as long as they are compatible to your motherboard. You can even mix different speeds, but there will be a trade off in the form of underclocking. Example: if you use a PC2700 module and a PC3200 module the PC3200 will only run as fast as the slower PC2700.
RAM is one of those items that you get what you pay for, if you want quality stay away from value RAM, I personally like Crucial and Corsair. Both of these manufacturers have a means of determining what modules of theirs will be compatible with your motherboard, they will tell you the number of expansion slots and the maximum amount of RAM you can use in each slot.
Edited by dc3, 19 June 2007 - 09:47 AM.