1. What's the difference between unbuffered and registered RAM?
Registered RAM has error checking built into it, therefor its very stable. You should only be buying this is you're going to be putting it into a server. A gamer, casual user, or even an enthusiast will only need unbuffered RAM. Overclockers won't touch buffered RAM.
2. Whats the cas latency thingy? and the numbers 2-3-3-6 or something? while others have like 3.
CAS latency relates to how your memory is accessed, and how often. This link here is much more helpful:
CAS Explanation. The other numbers (3-3-6) also relate to latency in the RAM, lower is better.
3. How does dual-channeled RAM work?
Dual channel RAM requires two sticks of RAM, each with thier own bus to the northbridge (I think). Basically it provides more memory bandwidth. Unfortunately, most processors can't handle that much bandwidth so performance increases are only about 5-10%.
4. what's the difference btwn Corsair XMS ram and Corsair's normal "value" ram?
Marketing spin. Seriously, the XMS stuff is the handpicked RAM that'll perform the best. They test it to make sure it'll perform better than average, then jack the prices way up. The "value" RAM is excellent for the average user, and even pretty good for a casual overclocker. Hell, I use OCZ Basic Series RAM and its running at 400Mhz right now. Only $80 for 512mb.
As for the AMD 64, I have heard that the NForce3 solutions have been pretty dissapointing... but thats about all I know.