Normally there is a setting to "Auto detect memory". which I'm sure your BIOS does... since your post shows 3GB : 1 usable.
Memory diagnostics, will run a gamot of tests on your memory chips to make sure they are 100%. which is located under Admin tools.
some times different manufactures set there mobos to access RAM at different times. which only makes so much usable by the system.
I think that is adjustable in the BIOS under memory. your manual should tell you how to check or adjust.
some references to same problem:
how to adjust shared video memory - Tech Support Guy Forums
2 posts - 2 authors - Last post: Feb 10, 2008
how to adjust shared video memory. i have vista, p5n32-sli deluxe mobo, 3.4 pentium d 4gb ram (2.8 usable 32bit) nvidia 8800ultra, ...