As to an OS: wait till Widows 7 comes out in October and get the 64 bit of the best version you can afford.
XP 64 is/was a Server OS. Do not get it unless you want to run old servers.
The RAM issue: whether Dual channel of Triple channel: Use it.
Also use a minimum number of sticks. For example the best 4GB is 2 x 2GB in Dual channel.
The best Triple would be 3 x 2GB.
Again such RAM should be purchased in packages to ensure their exact match.






. Actually its the drivers that