OK I'm on a roll here blogging as my brain is full up on the MCA course. Here's a good tip on how to calculate the MAX_SERVER_MEMORY
a) calculate worker threads (assuming set correctly, as default max 255 may be too low for huge beasts)
select max_workers_count from sys.dm_os_sys_info
b) Figure out thread stack size
select max( stack_bytes_committed )
c) guess how much ram to leave for OS/apps and MemToLeave
MAX_SERVER_MEMORY= TOTAL_SERVER_MEMORY - [ Os/Apps Memory] - [Threads * Thread Size]
so an x64 with 1024 threads and 64 GB ram could have MAX_SERVER_MEMORY of 58 GB.
On the IA64 platform this would be 56 GB as ThreadSize is bigger.