If you only use a computer for basic web browsing and emails etc (aside from games), then my advice is to just get a Mac Mini and an Xbox 360 (or PS3... but I'd lean towards the Xbox personally). Yes, I'm trying to convert you to console gaming instead. Bit off topic... but it is a complete waste of money spending lots of money on a high end iMac that doesn't even have enough horsepower to play today's games in its native resolution (There would be very few recent games that the iMac's 4850 can run at 3.7 megapixels), let alone a few years down the road.
You can buy a VGA cable for the Xbox and hook it up to your computer monitor... voila! Gaming with the comfort of a controller, and without all the hassle about upgrading PCs, installing games, worrying about system requirements, etc etc. And the $800 Mac Mini is a fairly capable computer that should last several years, saving you a lot of money over a gaming PC.