Is this ordinary Win XP or Win XP Embedded ? If XP, I would be quite concerned with the remaining life cycle for XP, the Win XXX Embedded versions have had at least in the past a longer life cycle (e.g. no W2k Embedded, so NT4 Embedded lived for quite a long time).
I do not know about the processing power requirement for your QNX system, but usually the x86 embedded boards have a much longer life cycle then ordinary desk top boxes. However, the RoHS requirements may prematurely terminate the production of some older non-RoHS boards due to lack of non-RoHS components.
For the user interface box, you are still going to have to replace the box due to the increasing resource requirements of future Windows operating systems.
At least for the QNX box, you should be able to extend the life cycle and thus reduce the requalification costs, with suitable choice of hardware, especially if you do not need the absolute top performance for the application.
When there are some severe realtime requirements for a system running a general purpose OS (Such as Windows or Linux), the usual practice is to use a real time extension, which in practice is a RT-kernel running the RT application and then running the general purpose (Windows/Linux) OS as the null/idle task in the RT-kernel, using the unused CPU would have otherwise been used by the idle loop.
Do you absolutely need the newest hardware for performance issues or due to end of production for the older card ?
Paul