PPTP FWD and/or NAT?

Dear All

I just see the pptp_conntrack_nat helper module. I think it should
track the connection that get NATed for PPTP client access to outside

#

               pptp_call
           ---------------->     do NAT   ---------------->

(a client)------------------->[linux-box]------------------->(a vpn server)

# Figure-1

So I can just use one client to use vpn access to a server at a time. A question here: How about, e.g., using two client to access two
seperated vpn servers on the outside?

The second scenario:
#

               pptp_call
           ---------------->         route         --------------->

(a client)------------------->[routing-linux-box]------------------->(a vpn server)

# Figure-2

Let's assume another scenario, If we have some clients with valid_ip
that don't want to get NATed(I just filter in FWD). So what should I accept in FWD if I want to grant their access to outside using PPTP protocol, that they will never face any problem? Are there specific ports to get open? or we should wait for another conntrack helper module to pass pptp traffic?

at last
PPTP FWD and/or NAT, which would be better?