Hi,
With regard to TG2 slowing down due to a screen saver, I haven't noticed this with TG2 but I also haven't often been in a position to. I do know that on the Mac someone had a problem with the SETI screensaver kicking in and taking a lot of processor time from TG Mac v0.9 on long renders, so I added a preference to stop screensavers kicking in or the computer sleeping when it was left alone to render. Funnily enough, just the other day I was thinking about adding this to TG2. A processor intensive screensaver will definitely take time away from TG2 if it starts up.
With regard to TG2 using less RAM when you start other apps, well on OS X at least I'm guessing this is because the OS is paging out memory used by TG2 from RAM to the hard drive, to make some space in RAM for the other app. TG2 is still using the same amount of memory overall, but less of it is in RAM and more is on the HD. It does allocate some big chunks so I'm guessing during a lengthy render it might have big areas that don't get touched for quite some time, and the OS pages those out of RAM to the HD when it feels there's an opportunity, such as when a new app starts. AFAIK TG2 isn't doing anything clever with the memory, because so far it doesn't seem to much clever with memory at all :-).
Regards,
Jo