(notice that it's info about a patch for Vista included in SP1, so this thing or a better one is likely included in any Vista or Windows 7 system you happen to have and you don't way need this patch, but the link has worth info)
http://support.microsoft.com/kb/940105/en-us
Does this have to do with the said "BF3 memory leak" and/or some issues reported in this thread?
In summary (and if I've caught it right), all DX9 and some DX10 and DX11 games allocate, for their own use, as much application virtual RAM (that is generally limited to 2GB in 32bit OS's and in 32bit apps running in 64bit OS's) as video RAM, not clear if gfx card's inconditionally (3GB in case you have a 7970 with 3GB VRAM) or only the chunk that gets actually used by the game. In DX10 and 11 there's a similar allocation done by the OS in the OS virtual RAM space (generally limited to 2GB in 32bit; application and OS virtual RAM together amount 4GB in such systems), so that modern games don't need to maintain a copy of the VRAM in the application virtual RAM space, although some still do.
What the patch improves is that these costly and potentially impossible allocations don't consume actual virtual space, unless the reserved/allocated memory (not sure about nomenclature and concepts here) gets actually used, or something like that
.
Notice that these memory reservations/allocations/uses are independent from the use of physical RAM (that's up to the "virtual memory engine" distributed among processor's hw and OS code: the "defined" virtual RAM will be either in available physical RAM pages or paged out to disk) and the physical management of VRAM, that in 32 bit systems is done through a reserved "window" of upper physical memory addresses (upper = close to the 4GB boundary but below it) whose corresponding physical memory turns not usable (as ordinary physical memory at least). I believe that in 64bit systems it's done the same way although with the said window in way higher addresses (notice that you'd need over 4 billion times 4GB to fill a 64bit map!). Such "stolen" memory window is smaller than high VRAM sizes (even smaller than 1GB let alone 3GB) and I dare guess that independent from VRAM size? 512 or 768 MB no matter if 1024, 2048 or 3072MB VRAM? (at least my own 32bit OS's report 3318 MB usable so it's 778MB of physical RAM not usable here, with a 1GB card installed).
Crossfire: doesn't a CF/SLI config have only as much "application and OS usable" VRAM as one of the cards? Of course you'd still have to load all with textures etc, but wouldn't that affect to the window of physical memory addresses stolen from the physical map, and not to the application/OS virtual RAM?