rtz, you can't get the AR in those 3 calibers with a single receiver (not thatyour post is incorrect, just FYI to the OP who may not know).
If I had money burning a hole and it had to be another firearm I'd hit the shops hard and find a used centerfire in some caliber new to me to have fun with. My last buy was a duplicate in 30-06 Springfield (back up) and at less than 1/2 the original price it's a great shooter.
Aside from the satisfaction level of taming someone else's reject, I still had money to spare for Christmas gifts (3 kids) and another purchase down the road. Had things not worked out, I figure I'd be out $50-$100 to trade.
Looking at the calendar, if memory serves me, there's a certain GI pistol that turns 100 in 2011...(subtract 100 for model # if you don't know;-). I'd probably be scouting special editions from a short list of makers for something that catches my eye. Of course if it were me, I'd bee looking for 3 to hand down to those kids, another reason to buy only when you know what you really want to spend it on.
Good luck with your decision.