I have a dual booting system with 32 bit XP on one hard drive and 64 bit 7 on the other. With only 4 gig of RAM, I found that maps load much slower in 7, so I boot into XP to play FH2. I don't even have BF2/FH2 installed in my 7 installation anymore. I do pretty much everything else on my computer in windows 7 since I generally prefer that OS. I only use XP to play FH1 on pixelfighter and FH2.
windows xp 32 bit can only use 4gb ram + 4gb as ramdisk or cache in hidden memory if you have more than 4gb ram
when I run FH2 in xp 32bit, only 1.8gb ram is used incl windows cache. Its possible to cache FH2 files in hidden ram with software utility. But I dont know much faster load times is if you already use ssd as hdd
have not looked into load times win7 vs xp yet :-)
To me its all about gfx and OS micro lag/stuttering.
Everything looks a little sharper and better in xp without vsync enabled when doing fast moves, like turning 180 or 360 degree in game