Solved Windows x86 and 4GB...and what about dual-channel on the mobo?

OK, for a change I have a question of my own.

I've googled around and read old threads on this forum and know all about x86 and 32-bit address limitations and hardware devices using memory mapping etc. etc.

So, I have 3GB at the moment, two 1GB sticks and two 512MB sticks so my mobo can use dual-channel support on the entire RAM. If I decide to replace one of the 512MB sticks with a full 1GB stick, netting me a total of 3.5GB in the process, how will this affect dual-channel support? Would it still be active over the first 3GB, or be completely disabled?
Basically, would it be better just to go to 4GB and have four 1GB sticks?

Also, is there a certain hard limit on just how much memory (out of 4GB) the x86 version of Windows 7 can use even under ideal conditions? For example, is 3.5GB the absolute most I could hope for? Is it 3.25GB? Or 3.75GB?

Thanks in advance!!

As far as I'm aware, 3.25 GB seems to be the ball-park figure you can use with the x86 architecture.

Crucial's memory scanner will give you good advice as to the best configuration for your memory modules, but at this stage I would recommend 2x2 GB sticks, which leaves two sockets spare should you go to Windows 7 x64 and want more memory then.

RAM Memory Upgrade: Dell, Mac, Apple, HP, Compaq. USB drives, SSD at Crucial.com/uk

Thanks seavixen.

Won't be going to 64-bit Windows on this board anytime soon, by the time I do I'm sure it'll be on a new computer...this motherboard only supports a total of 4GB anyway.

I still hope to get an answer about dual-channel support when using mismatched amounts of memory in each channel...anyway, for a mere 256MB gain it doesn't really seem worth it (I was hoping for 512MB though)