32bit vs 64bit benchmark and <4gb ram

[skiv71]

hi, been testing centos i386 and x86_64 with a q8300 2.5ghz quad core and 4gb ram.

my first attempt was with i386 but i read that the 32bit kernel would only use 2gb but it seemed to run ok none the less.

tried today in 64bit flavour an noticed the ram is only showing as approx 3.25gb due to (i think) shared vga an other onboard resources.

i dont really wanna change the mobo, just wanna get the best out of the rig.

is it true that only amd are true 64bit cpus?

the 32bit, 2gb per process thing with the linux kernel, will that affect zimbra? i mean there must be more than one zimbra related process?

is there a way to get a definitive on performance? some internal test etc?

i really wanna get this box into production, less than 20 users.

thanks, neil