Actually it depends on the server specifications and the transaction volume to be processes. If you have sufficient CPU, RAM and DISK a NAS service MIFGT work on the same box. Basically. If you run a 32bit environment you have to be careful, as you might encounter problems with adressing RAM. Even on 64bit systems older NAS run into memory-allocation-problems (WoW Mode).
(see Roberto’s posting)
Thus, to have sufficient fault-tolerance IMO it’s wise to have separate machines for SQL and NAS.