Is dual channel good? Does it do anyting? And does my ABIT NF7 V2 supposrt DUal channel? Would getting 2(256) be better then 1(512)? thanks in a advance.
Dual channel is good, mainly in Pentium 4's. P4's thrive on tons of memory bandwidth, which is what dual channel gives. AMD's don't need it nearly as much. If you have an AMD setup, and you can save some money, then you might as well get 1 stick of 512. Of course, dual channel can't hurt anything, and will give you a few extra percent performance.
If you have a P4, go dual channel no matter what the cost, if you have an AMD (which you obviously do), then you might want to save some money with 1x 512 unless dual channel is just a few bucks more. It can't hurt anything anyway.
For an Athlon XP rig i would suggest buying 1 512MB stick instead of 2x256MB kit because of the fact dual channel does next to nothing on an Athlon XP motherboard. Buying 1 512MB stick will leave you more room to upgrade.