Not quite. Any 32-bit system (XP, Vista, Linux, etc) supports a maximum of 4GB or RAM. The reason that we all see less than that is largely due to our video cards.

If you have a 512MB video card, then you'll only see about 3.5 GB, if you have a 1GB card, then you'll only see 3GB. And if you have 2x 1GB video cards SLI'd together, then you'll only be able to use 2GB out of your 4GB of RAM.

That's how it is for PCIe anyway. AGP video cards are worse.

Don't think that switching to a 64-bit system will fix this either. It won't. The memory hole will still be there, it's just that you're now able to buy RAM that goes past the 4GB address limit and is able to be used by the system again. A 16GB 64-bit system with a 512mb video card will still be missing the memory between 3.5 and 4 GB.

This is all due to memory mapping and that initial 640kB memory barrier on 16-bit systems. Remember that?

It's better explained here: http://www.dansdata.com/askdan00015.htm