Funny enough, the Xserve 1,1 uses
JEDEC-standard DIMMs
, something which seems prohibited in your Mac Pro system.
Considering the size and the amount of heated components inside, I must say it's a fairly silent 1U server after all.
I work as an IT-Consultant and deal alot with HP Proliant based Intel systems, whereas 1U servers *always* tend to emit loud screeming noises, just because of cooling requirements. The Woodcrest based HP Proliants would be equal to Generation 5 (G5) units for comparence and I know that neither of the G5-models is anywhere near silent. There's just too much heat coming from both CPUs and the FB-DIMMs.
The other Xserve 3,1 is Nehalem-based and from what I know they would be equal to HP Proliant G6-series. I haven't dealt much with those generations, but I know the later G7-models are damn near whisper quiet, for beeing servers. Also I believe power consumption is considered much lower on the Nehalem (and newer) generation CPUs.
Nehalem (and newer) uses DDR3 memory which doesn't even require heatspreaders, something that also indicate lower power requirements = less heat.