I just did some googling. It seems that if you have FSX Deluxe, it comes with something called the FSX SDK. The FSX SDK has some tools in it that let you import .x files. AC3D has a .x exporter. I haven't tried it personally, but in theory this should work.
Here's a tutorial someone wrote on using the FSX SDK to convert .x files. It's for XSI, but the steps should be the same for AC3D:
http://adventuresinmodelling.spaces....9F8E!166.entry