I've tested Windows 7 x86 in a bunch of motherboards and notebooks and I always got 3,25 GB. or 3,5 GB. of usable RAM.
Anyway, 64 bit edition of Windows 7 have few little incompatibility issues compared to the 32 bit edition that can be a problem for just a few amount of users out there. So, if you're not some of that few users, I highly recommend you to install Windows 7 x64 instead of x86 edition.