i am looking to buy some more ram for my system. i need ddr 400mhz 3200. i see that some have a CAS latency of 2, 2.5, 2-2-2-3, what does this mean? will they all work with my system?

MSI 865PE 478
P4 3.2 HT

The technical answer is here:
http://en.wikipedia.org/wiki/CAS_latency

You can find exact compatable memory at www.crucial.com, use their utility to find the match by mainboard or run their utility on a live PC to show the specs on the memory you currently are using so it will match the current.

just buy pc 3200 184 pin. it will work. make sure it is dual channel, and as a rule of thumb, the LOWER the latency the better, as long as your running at the best frequency.