INSTALL EXCH 5.5 & PROXY

HAVE PDC HOSTING APPS AND MS MAIL POSTOFFICE FOR 25 USERS. PLANNING INET CONNECTION THRU DSL THRU A NEW BDC FOR MAIL USING EXCH 5.5 & MS PROXY ON A SEPARATE BOX..
1. CAN I USE PROXY AS AN EFFECTIVE FIREWALL ?
2. DO I HAVE TO UPGRADE WIN9x MAIL CLIENTS TO ATTACH TO EXCH ?
3. WHATS THE PROS & CONS OF PLUGGING DSL OUTPUT DIRECT TO BDC THRU PROXY ? REQUIRES 2ND NIC TO GET TO SWITCH ?
4. ONE APP RUNS ON IPX/SPX ( NOT A ROUTABLE PROTOCOL ?). I TRIED A SECOND NIC AND I GET A BLUE SCREEN WHEN THATAPP STARTS. ANY ALTERNATIVES ?