Arduino Serial input and output for RC car.

Hi all, im new to arduino, and planning a robot like this:

HP IPAQ(which has a program running on it) serial port(rs232/db9) communicate with arduino, when Arduino get serial data, it tells the RC car's driver board to move forward etc. As far as i know it use rx/tx to hook up the RC car's onboard chip. please check this article(im sure u have read before): jbprojects(dot)net/projects/wifirobot/

Due to i cannot find a Arduino Serial or freeduino Max Serial like board around the web(countrywide), im trying to use a usb interface Arduino with a converter (Max232). But the converter should use Arduino's onboard rx/tx as you all know. how can i use another rx/tx to communicate with the driver board. Mayb other onboard IO? Which or any? and how's the code like. Can i do that like this? Or what more should i get to make this work.

im quite new, please b gentle ;D