paul3200 wrote:
i am thinking of building my own PC and was wandering what is the most
ammount of RAM any virsion of Windows can read

A 64 bit OS is likely to handle whatever desktop motherboard you build.
Even a Core i7 full of RAM (24GB).

http://msdn.microsoft.com/en-us/library/aa366778.aspx

For something like 32 bit WinXP, you have 4GB of address space
(since PAE isn't available), minus the space used by the video card
or cards. For example, on some older systems (A8N-SLI Deluxe with
two 512MB video cards), you might see 2.75GB of RAM reported in
Windows, even though 4GB of RAM was physically present.

At the time, memory was expensive enough, that people would be tempted
to buy 2x1GB + 2x512MB and populate 3GB total, so not much of
it would be wasted. But at least in the case of DDR2, the memory
is so cheap now, you just buy 2x2GB and don't worry about it.

The purpose of the matched pair, is to run in dual channel
mode. Buying 2GB + 1GB sticks, while more economical, would
not give as high a bandwidth. While some motherboards can
run a mixed set like that in dual channel mode for part of
the address space, it really isn't that desirable to have
memory that has different performance levels as you move
through the memory space. (Intel calls that capability
"flex memory" in the chipset datasheets. Nvidia may have
been the first to support a mode like that.)

I bought a nice 2x2GB DDR2 kit for about $60, and I'm not going
to guess what the price is now. Memory prices have gone
up 50% in the last few months, but since memory was dirt
cheap then, it is still a bargain.

Paul