Not sure if this has been resolved, but this is a common problem for any 32bit O/S... The issue is not that the O/S supports 4gb of memory or not (it does), however, it's the addressable limit that is 4gb total! Now, (i think it might only affect PCIexpress video cards) but Video Card Memory gets counted as part of the addressable limit... therefore, the Original Posters (OP) specs were 4GB memory - 1.5GB (2x768 Cards) = 2.5GB of actual memory addressable by the O/S... Another example, 4GB memory - 512Video = 3.5GB usable RAM.

Most people with 2GB ram plus 512 video card don't notice this problem cos total memory is 2.5gb...

Now, in saying this, i think there are some boot switches you can add to VISTA startup /4GBT (which re-maps the memory addressing), otherwise, there is no real workaround yet.

Also, x64 Vista is supposed to support greater than 4GB, but I think it is still limited on a per-application basis. (this needs more investigation)...

Also, even tho Vista sees 2.5GB, i think some applications can see through the 'soft-limit' and access more ram (i.e. Crysis might use 3GB instead of limited to 2.5GB)... ?? not sure though.