2nd hand iMac question, duo core and C5

Hi there
Recently my MacBook pro died, tried to get it repaired but the cost now is really not worth it. I was really happy with its performance with cubase 5. I was going to upgrade to 6 and buy a new quad core iMac but I really don’t think I need to go down this road just yet and am thinking of saving some money and getting a second hand iMac. Can any one offer any advise on what to go for, and what to keep away from. I will be rnning it with an audio ire 12, and no 3rd party Plug ins, just steInbert stuff.