PIX or 1721 Router

I am looking for some knowledge on which device I would be better off using at my place.  I am currently using a PIX 506E, but I have a 1721 router sitting unused on my desk. The PIX is running version 6.3(4) and show mem returns
Free memory:        13997288 bytes
Used memory:        19557144 bytes
-------------     ----------------
Total memory:       33554432 bytes

The 1721 is running IOS version 12.3(9c)
and has 32 megs of flash memory.

I am at a non-profit, and received this equipment through a donation program, so purchasing different equipment is not an easy option (purchasing different Cisco equipment is probably out of the question).

Currenty I have a the PIX 506E set up in the main office.  For the most part its DSL connection is used only for VPNs, and as a backup if our other internet connection goes down.
I have 3 site to site VPNs running through it to remote sites also with PIX 506Es.  
We have a Windows 2003 domain, and these 3 sites have local domain controllers and have about 20 other computers on each LAN.  The main office has about 100 devices on the LAN.

I also have two site to site VPNs to smaller offices with no local domain controller and about 5 PCs at each site. They both have Cisco 851 routers.
All of this has been very solid for over a year - never any problems with the VPNs or Cisco equipment.

Now I would like to set up site to site VPNs with our remaining 4 offices all with 1 to 5 computers and no local domain controllers.  This would be a total  of 9 VPN connections to the main office.  I would also be using Cisco 851 routers at these locations. (Unless there is a better choice at a comparible price?)

So my question is which device should I use at the main office?  Are 9 VPNs pushing it for the PIX?  Would the 1721 handle this better, or is the PIX better equiped for this kind of use?