i was no real time "stricto senso" but i studied a OS/2 based adaptative controller(wich works with quite complicated linear algebra)
a simple i386 + 16 MB RAM just achieved to make 8 units , 4in-2out controllers (laguerre polynomial based) handle a huge bleach production plant. (i've also studied a version for crisp friing, and you can't imagine how two potatoes could be different)

Of course to have the same response time under win 2000 you would have to have a nice Pentium 4 with 256MB ram (and pray for your southbridge no to be buggy, introducing pseudo random lags when accessing too much memory,wich is not that rare nowdays), that means : "Buy some industrial PC stuff, forget other vendors"

microcontrollers are basically micro-computers, the bigger version won't be as much responsive, but will have enough power to calculate what is needed to compensate a system error.

A mixt system containing a computer and a controller based system is also interesting if you wan't to have top performance.You can or example use you 8051 as a light PID controller to have faster response

When designing your system, please consider looking at a numeric analysis book to have good interpolation methods.

Last point, for the computer solution is taht you can easily find hardware, with drivers or programing interface to shorten significally dev time.
The whole probleme is "How much does your time cost"

Regards,
Benjamin