VPN and ISA2006 ?

Hi,

We've two LAN which are connected via a VPN.
On each LAN, there is a server Windows 2003R2 SP2 (AD, DHCP, DNS...), both
servers manage the same Domain.

In the objective to manage the Internet access of all workstations (Windows
XP Pro SP3), we plan to install an ISA server.

Because our VPN connexion is slow and down sometime, we would like to know
if it's better for us to install one ISA server only, or one ISA server on
each LAN?
Is it possible to have two different ISA servers on the same Domain?
Does the ISA server have to be member of the Domain?

We haven't ISA server yet, and we would like to know if there is a lighter
Proxy solution which can regulate Internet Access on a small LAN (like an
OpenSource software)?

Thanks,
Chris