Not sure about enabling it as RAM, but there are instructions for using the the non-usable memory as a RAM Disk, which does supposedly increase performance, but i've never had enough RAM in an XP machine to warrant trying.
THIS IS NOT FOR Win 7, sadly, just XP. as i am new member (joined to post this) i cannot post links, however the title of the article is:
Making Use Of Non-Addressable Wasted RAM On 32 Bit Systems can be found under "/RAMDisk" on the site megaleecher . net
If you own a 32-Bit with more then 3 GB of RAM you must be knowing that your operating-system can only make use of roughly 3 GB RAM and rest remains unused because of , today I will be showing how you can make use of this wasted memory improving your computer speed significantly.
The best way to utilize this wasted memory and speed-up your computer is to use it as an
RAMDISK, storing frequently accessed temporary files like system pagefile, temp-files and web-browser cache, since I/O operations to RAM is significantly faster then disk you will greatly benefit from using RAMDISK as a temporary file storage.
I tried a similar techniqe to the OP, but couldn't succesfully boot with my custom kernel.
Hopefully this works better. I'll report back on results.