All DDR Ram behaves pretty much exactly the same way. And even that's pretty much identical to SDRAM, except it's internally clocked twice as fast.
The various numbers (CL2.5-3-3-6-1T, for example) refer to different timing delays inside the chips. RAM is organized into rows and columns internally, and a certain amount of time has to pass between accesses on successive rows, or when switching from read to write, etc. Those numbers define how long the delays need to be. The lower they are, the faster the RAM is running. Better (read: more expensive) memory chips have tighter timings and are able to run with faster settings. The very best stuff runs at CL2-2-2-6 1T, though the SPD (built-in detection) speed is usually something like 2-3-3-6 1T, which is more stable. Very high speed ram (DDR500/533) usually runs around CL2.5-3-4-4-8.
The most important number is the first one (CL = Cache Latency). CL2 is the best, 2.5 is pretty standard, and 3 is slow. The other numbers have a much smaller impact on performance -- The difference between 2-3-3-8 and 2-2-2-6 is usually like 5% in memory bandwidth testing, and maybe 1-2% in real benchmarking.
Also worthy of note is Intel's PAT (Performance Acceleration Technology). This also tightens up certain timings in the memory controller to increase performance, but it can cause instability with lower-quality RAM.
As far as your particular motherboard... unless there's a BIOS you can get that increases the max FSB, you're probably stuck at 400. However, most NForce2 Ultra boards can run higher than that, so maybe you're just not looking in the right place? Check the advanced screens in the BIOS -- you may have to flip certain settings to "manual" to be able to overclock it past there.
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