Crucial. Every hardware endor I've asked says that Crucial make the greatest RAM ever, at least for my purposes...!
It depends alot on your motherboard what it can take (I am assuming DDR), but for sheer speed you want unbufferred. For the ultimate in paranoia (for running mission critical applications) you want ECC Registered bufferred RAM, which is slightly slower but every access process is checked to make sure there are no errors.
For a compromise between reliability and speed, go with bufferred unregistered.
If you go to the
Crucial website their "memory finder" will find out what mem your system can take. You'll either need the make of the mobo you're using (if you BYO), or the full name of your computer.
e.g. Personal computing > Dell > Dimension > Dimension 4500 will give you a selection of RAM for that computer.