I bought the 2010 2.8 quad Mac Pro with the 5870 gpu about a year ago, when it came out and was considerable more value for money. Hex upgrade was too expensive for me. It's been a great machine that is very fast for everything I throw at it - mostly Adobe CS, Cinema 4D, some video encoding and gaming. So far every game runs great at high/max settings at 2560x1440 on my 27" Dell monitor.
I'l just list a few:
OSX: Starcraft2, Dragon Age 2, Portal 2, Left4Dead 1+2, ... etc.
Windows 7: Total War Shogun 2, Metro 2033, Fallout New Vegas, Batman Arkham Asylum, Mass Effect 1+2, ... etc.
Sorry, can't think of everything right now - because I'm not at home.
That said, I wouldn't advise you to buy the same machine right now. One year ago its value for money was ok, but now it is not anymore. You mentioned you have a 2009 iMac, why don't you wait for a couple of months for the next Mac Pro revision?