I believe video RAM is mapped into main memory, but there seems to be something else taking up the recognisable amount of RAM.  I have 4GB with a 512MB video card, but 32-bit or 64-bit with /maxmem=4096 only recognise 3.25GB (I use hibernation, so I have to limit myself with the maxmem switch).

For 64-bit vs 32-bit - advantages/disadvantages:
+ Slightly faster performance
+ More than 4GB addressable memory
- Drivers, mainly for older devices, may be hard to find
- Application compatibility - mainly for apps which interact with low levels of the system (just find 64-bit versions of these)
- No 16-bit support (use DosBox for DOS apps, or VirtualBox for Win16 apps)
- WOW64 virtualisation quirks? - unconfirmed I guess, though I've had some speed issues running ZSNES on XP64 - otherwise, all other 32-bit apps I've run have no major issues
- More memory/HDD space usage (but if you've got plenty of both...)

Would go 64-bit for the performance if you can get the drivers.