I'm in the exact same position, how old is yours? Mines 6 years old this month. As soon as I got the iPad 2 my iMac use was completely destroyed. Can't make up my mind if I should get the rMPB or just hold on to the iMac for another year.
Mine is about 5 years old. It still works great without issues but i find that new apps are much more demanding and my core 2 duo doesn't keeps up that well anymore. And i can forget about editing HD videos as its painfully slow. In the end i want to new computer but not necessarily need one.