It sounds kinda of tempting, but ill still hold down to the Quad. Further the Mac Pro is still an early model, with defects, which may get worse.
While your Quad is running fine. If u happen to make the swap and something awful does happen to the Mac Pro.....ull be cursing!
I once had an offer for my G5 2.0 DP, double the sum which i bought it for, but just couldnt let it go, now i am happy that i made the decision!