For me, it's a bug if Photosynth runs out of memory (too many pictures or too big pictures) and does'nt better handle virtual-memory but ... my last experience is: trying a synth with 260 pics (1280 x 960) on a 2 Mb XP computer
I ran out of memory in the evening after using Photoshop, Open-office and synthing 4 times or more.
On the next day, I took 12 pics away and I could synth, and I was quite surprised; so I restarted the computer to get back all the memory, and I could easily synth the 260 pics!
So, I think there is still the old Windows'bug not releasing the whole memory after ending a program; and maybe a Photosynth'bug not releasing the memory after publishing a synth.
If this experience could help somebody ...
By the way, the synth was http://photosynth.net/view.aspx?cid=7a5ef951-d15b-4d83-839c-5013b01e4726 just a nice simple new cathedral in the collection ;-) have a nice visit!