Upgrading ram can be a painful experience so just to avoid that and assuming price is low it can make sense to go for 16gb. So a nice overkill and ramdisk can always make use of "too much" ram :)

Look for places.last_vacuum

No I am not. But from 3.6 I am confident that places.sqlite is vacuumed https://bugzilla.mozilla.org/show_bug.cgi?id=512854 Session restore use other files or so I guess, not sure it was relevant to bring up. Programs like CCleaner vacuums all sqlite files in profile.

I have sometimes noticed a somewhat faster awesomebar after vacumming but there is a long way to starting up Process Explorer to investigate. I don't think session restore cry for being vacuumed can cause such problems. Corruption probably can.

Sqlite stuff used on 4 must be the same as on 3.6 right? Just a type of library? Then it is difficult to understand 4 feels so much faster and more responsive when it also does much more. Endless history based on automated detection for one. Like now there are no limits at all. Disk cache also 250mb. I think truth is 3.x sucks with this but just have never seen anyone complaining about session restore being such a pain when it does what it is supposed to do. Must be very weak computer then - most never notice, a non issue but when sqlite files are involved much can happen.

"Our cache code is ancient and sucks. " from a Mozilla developer last summer :) Not like they don't know though he mostly talk of file fragmentation. http://blog.mozilla.com/tglek/2010/07/22/file-fragmentation/ see under Preliminary Conclusions. More from their wiki https://wiki.mozilla.org/Firefox/Projects/Places_Vacuum#Vacuum_on_idle no date but I guess early 2009. Things move slowly with Mozilla, they talk and wiki away before they implement.

23.09.09 was birth of mini vacuum, see item 17 http://forums.mozillazine.org/viewtopic.php?f=23&t=1497415&hilit=vacuum