1- I'll say that is your internal network is highly sensitive, you'll introduce a risk by using a XP workstation in the middle of internet/intranet.
Even if the PC does not have ip forwarding active, an attacker could gain admin access to the PC and use it to bounce into ur network ... (I would not do that...)

2- Be aware that windows can have just a single default gateway (0.0.0.0). There is no problem by using sveral NICs (I think its up to 4 for many mother boards) but the default gateway will have to be on the internet side. For the internal side you'll have to configure statically your internal routes (it could be aggregated in 1 or 2, I guess!).
TO configure statics routes on windows use the route add command from the dos prompt.

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