It's a Windows 32-bit thing...you won't get that with XP 64-bit or Vista. It has to do with the video ram occupying, the same address space as the MB ram.
With 4GB of ram installed and 2X 7800 256's Windows will only report 2.75 GB...in your case it would report 2.25GB.
But yours is the first case I've seen of the video RAM eating into space below 2GB. Didn't think that could happen.
Look in the ASUS site under the A8N32 FAQ...it's the first item.
Just out of curiosity, add the /3GB switch to your boot.ini...Widows addresses 4GB of virtual ram...by default it keeps 2GB to itself and releases 2GB for use. The /3GB switch changes that so it will release up to 3GB. Just curious if that will force it to manage the RAM differently and give you back your 2GB.
Microsoft KB article about /3GB