I'm not sure that Real Time RTX is the best solution for you. The Real Time OS would just be running under Windows. That option is more for adding determinism to your system then increasing the speed and from your post it sounds like you want to increase the speed of your app.
Real Time ETS with a faster processor would be better as then you'd be using a PC dedicated to the real time OS, without any Windows interference.
There is only support for RTX under DAQmx Base (a stripped down version of the DAQmx driver) which doesn't support M Series devices. DAQmx has support for RT ETS however so you could use your PCI card in that system no problem.
Finally, whether your LabVIEW 7.1 code is compatible with Realtime will depend on how your code is written. If it is architected efficiently in windows then it probably will work, but labview for windows is a completely different environment to labview for RT. They do have alot of the same components so like I said, it probably will work but I couldn't say they would be fully compatible.
Hope this helps,