The last released version of MTSX RSS was R6.08, December of 2005. It runs fine on (then current) hardware such as P4 2.6GHz single core CPUs, Core 2 Duos, etc but the key here is booting to a real DOS O/S, and a hardware serial port.
I've got a Dell Core 2 Duo with 4GB of RAM and a small FAT-32 partition for DOS based RSS. I boot from a floppy. MTSX R6.08 runs flawlessly on it, using real OEM RLN4008E RIB and OEM cable.
No need to search for some stone age 486 for MTSX. In fact, it will actually run much faster on a P4 class machine especially when you load large codeplugs. Paging through menus on a 486 in MTSX is painfully slow. Not to mention, finding WORKING 486 class laptops is becoming difficult, and when you do, often times they are being sold at collector's prices whereas decade old Core 2 Duo based laptops with RS-232 ports are a dime a dozen and (sans batteries) are usually perfectly functional.