Memory Question and will this work?

My new mobo will take 4Gbs of RAM at 400Mhz:
(either a ASUS A8N-SLI or A8N32-SLI), but I hear Windows will only read something like 3.5Gbs...

Does the other .5 Gb of Ram actually go into use anywhere or is it completely lost in the initial setup allocation?
I was gonna buy 2 of these Kits:
http://www.valueram.com/datasheets/KHX3200K2_2G.pdf
- 2 GBs Dual-Channel Kit
- DDR-3200 (400 Mhz)
- CL2 (2-3-2-6-1) DIMM
If I fill out all 4 slots (1 Gb sticks each) correctly from 2 of these Dual Kits, will it run 2 Dual-channels and hopefully at the rated DDR-3200 speed and/or latencies? I'm more concerned about running 2 dual channels and the speed that its rated at (and that I'm paying for) than I am about the latncies..

I don't know about the memory limits, but you might want to use a 64 bits version of Windows (with a 64 bits processor, Athlon 64/FX/Opteron highly recommended) which will surely use all 4 GB.

As for dual channel, yes, it should work, even if the OS can't use all of it for any reason.