I'm in that boat. I'm waiting for the new Mac Pro while still chugging along with my 6 year old G4 DP800 (1.5g RAM, 256mb Radeon 9800 Pro). The computer is showing its age and I'm more than due for an upgrade, but until the new machines come out I'm staying busy with FCP 4.5 and the latest version of X-Plane, which both run decently. Of course I'm looking forward to the huge performance gain I'll see with the new rig, and the first thing I'm doing is upgrading to FCS2.
Cool to see some X-Planers on here. X-Plane 9 should be out in early '08, so when you order your MP go for the extra processor speed if you can, that's the best way to help your framerate with that sim. At this point X-Plane only takes advantage of two cores, but that may change in future versions. For now, quad-core is more than enough.