Have the dreaded Graphics Card overheating issues. I use SMC running full blast to stop the thing from crashing.... I realize that complex bits of kit like this can go wrong but its Apples stubborn refusal to acknowledge the issue that upsets me.... there are a million threads that describe the same behavior in early 2009 24 inch iMac's... To be honest, i am nearly ready to order a new 27 with SSD, I am just finding it hard to part with my money with the knowledge Apple are not looking after customers.
My early '09 24" iMac is still running fine although I've had the display replaced twice (once under warranty). My advice, skip the iMac and either wait for the refreshed mini or if budget allows get the nMP. Investing in a good display that you'll keep for years and having a standalone system is the way to go IMO.