For comparison, on a large project with a lot of soft synths (not counting Hollywood), my CPU runs about 15-20% and memory usage is about 4.5 GBs out of 12 while the song is playing.   By "a lot of soft synths," I mean:
 
• 2 instances of RealGuitar2
• 1 or 2 instances of RealStrat
• Ivory II Grands
• SI-Bass
• Session Drummer 3
• Possibly 1 or 2 instances of Kontakt for either Acoustic Legends HD and or Electr6ity
• Possibly 1-4 instances of Vienna Symphonic
• Possibly an instance of Play (e.g., FabFour)
• Possibly an instance of Fantom which resides in the V-Studio, although that should not hit the CPU.

I have a Win 7 64-bit i7 (8 core) with 12 GBs RAM.

This is the very reason why I recently upgraded my music workstation. I got plain fed up with freezing and unfreezing tracks all the time, and I eventually lost interest for a bit. I probably should have upgraded sooner, but I also ran out of "inspiration" at the time, and I didn't produce anything of note (no pun intended) for over four years.

Can I ask a personal question? How long have you had your particular set-up? It may be that, as the software has developed, and your collection of soft synths and stuff has increased, you need to take the plunge and get yourself some new hardware... Easier said than done, with ever tightening budgets. But at the same time, there are some great PC deals out there to offset any recession woes. 

Don't know if this is your cpu version or not, but a representative benchmark is here:

[link=http://www.cpubenchmark.net/cpu.php?cpu=Pentium+Dual-Core+E5800+%40+3.20GHz]http://www.cpubenchmark.n...Core+E5800+%40+3.20GHz[/link]

You are running at about 1/3 or less the performance level of an i5/i7. There is no way "around" this. If freezing
doesn't work, you'll need a newer mobo/cpu and some more ram.

So if I were to increase my RAM to say, 8GB, what would be the minimum (or least powerful) mobo/cpu model that could handle it?

Just reading between the lines - if you want the expense kept to a minimum, I think you're looking
at about $400 (about euro320 maybe)  for a mobo/cpu combo. Almost any newer i5 will work fine for you.
It will be a different socket than you currently have, so you'll need a compatible cooling fan also.
The 8gb ram should be more than sufficient. You can check the normal vendor web sites for combo deals,
and you might also look at AMD options (might be less cost). The question you just asked will likely provoke
a lot of response from people eager to let you know about their setup and how it works for them, so if
you check back a few times you should get some more specific info. Best of luck.