If it helps, I'm planning on my 2.8 iMac to last 4-5 years. And I have every confidence that it will. My old G4 PowerMac lasted 6 years (though for the last year or two it was too slow).
Why not buy/get the slower and cheaper 2.4 model with the 20" screen, put the remainder of your money in a high interest bank account or a gold coin...then in 2 or 3 years you will have enough money to buy/get the cheaper variant of the next model iMac, once you sell the existing one on eBay. It will still be worth something then.
The 2.4 will be more than enough and in 2 years time you get another new iMac