Is a quad-core i5 or i7 27" iMac a viable upgrade from a 2008 2.8ghz octacore Mac Pro 3,1?
It would be a lateral move in terms of processing performance. The following are scores taken from Geekbench (how fortuitous that they were right next to each other?):



As you can see, it's a slight downgrade in processing power, and I stress slight, if that is indeed your current model (3.2 GHz). The other variants (3.0 GHz for example, which can be seen if you scroll down the list) clock in at slightly lower numbers (~9000). While you would gain the use of an additional Thunderbolt port, the 6 Gbps controller, and a faster FSB, they may not offset the cost of buying a new machine.

It really doesn't look like it would be a significant upgrade from your current Mac.