Memory3

Recommendations:

Today's motherboards use DDR SDRAM or RDRAM (on some Intel motherboards only). RDRAM is expensive and the performance isn't much better in general use. Please note, for dual channel DDR RAM, you need two sticks of memory.
Corsair XMS memory is my number one pick. However, it's expensive compared to other brands. This memory is great for overclocking and getting low latency speeds, helping your computer to run faster. I would suggest PC3200 CAS 2 memory (C2). The specific model number is CMX512-3200C2 which has timings of 2-3-2-5-T1. Corsair's regular memory (not the XMS line) is also good and cheaper.
I recently started using Kingston's Hyper-X memory, which is a high performance memory at a reasonable price. I've been very happy with it and think it would be a great choice.
Crucial makes high quality, reliable memory, but it isn't always the fastest. At the time of this writing, they don't even have PC3200 memory available that runs at CAS2. It can be a little pricey, but their support, return policies, and free shipping make them a good choice.
A couple of newer companies to consider are OCZ Technology and Geil. These are both manufacturers of high performance memory, but I haven't used it much myself. Many users have been very happy with these manufacturers, although some have had problems with Geil (while others have been very happy with it).