Well; 2Gb RAM should be able to provide your GFX card with it's 768Mb. Add the 128; and a poweful 896Mb appears 'out of thin air'. However; if RTW for some reason ignores the Hypermemory, then it may think your other GFX card is about twice as capable as far as unit numbers are concerned. That would also explain why the laptop RTW advices you exactly half the amount of men on a battle field of the desktop version...

EDIT the 768 is shared memory; btw. ATI calls it Hypermemory; NVIDIA usually Shared.