I use Corsair XMS DDR, CAS2.0. Corsair makes the best DDR RAM in my opinion. It's not cheap, but it's designed specifically for overclockability and stability at low latencies. From what I've heard, Kingston is a good brand for RDRAM - But since you seem to be running AMD, you'll need DDR.
Lower CAS latency means faster access times and higher performance, but can result in decreased stability, meaning more frequent system lockups. You can manually play with it in the BIOS and see what your system runs well with. CAS latency is generally a bigger performance factor in gaming applications than clock rate. If you're trying to decide between PC2700 or PC3000 (333 and 366 mHz respectively) at CAS2, and PC3200 (400 mHz) at CAS3, go for the lower latency.
I've actually read the opposite of what you say you heard. Corsair recommends single 512 sticks, claiming that having a single stick is more stable than running two of half the size. I have a single 512 megabyte stick of Corsair XMS PC2700 CAS2 that I bought a year ago, I've had no problems at low latencies. I have no idea if nForce boards are special cases though. I have an Athlon 2000+ on an MSI KT3 Ultra 2.
