I installed FreeSBD amd64 recently (which despite the name is also for Intel processors) and was impressed. I never realised that it was a "tier 1" version, which basically means that it's expected to be stable and frequently updated like the 32 bit version. The only issue I had was with the onboard ethernet and occasional garbled packets but considering it was a Realtek chip (ie: usually crap) I wrote it off and installed an Intel PCI/X ethernet card in its place. Apart from that - and a different directory for compiling - it's virtually identical to its older 32 bit brother.
The mainboard itself only supports a maximim of 8GB but it's still nice to see more than the typical 3.5GB available...
Code:
usable memory = 8581259264 (8183 MB)
avail memory = 8301932544 (7917 MB)
Mem: 3226M Active, 4047M Inact, 341M Wired, 285M Cache, 214M Buf, 9024K Free
PID USERNAME THR PRI NICE SIZE RES STATE C TIME WCPU COMMAND
33887 mysql 11 4 0 5181M 3191M sbwait 0 91:10 11.28% mysqld
(mysql is the reason I tried this OS. Under FreeBSD 32 bit the maximum memory I can get it to use is just under 2GB.)