Few notes. Any broadcom 4702 based device could be easily modified to support 2 USB ports. But this is USB 1.1 only and work slow. If you want USB 2.0 take a look on to netgear wgt634. As for RS232 ports - you can use USB to RS232 device for this, but you will need to add corresponding modules to the kernel.
If you're thinking about linksys - think of GS version only - it's built with 200 MHz CPU (4712 - I don't know anything about USB support) and larger RAM/FLASH which is very important if you want to play games with custom firmwares. If you want guaranteed USB - think of ASUS or Netgear. But I don't know anything about custom firmwares for netgear.