I know this thread is really long and pretty old now.
But I just wanna throw in my 2 cents.
I'm and avid gamer the stock Bootcamp driver for the new iMac is NOT optimized for the HD2600, of course I have XP installed on my iMac. I also have had ATI Catalyst 7.9 Driver for the desktop HD 2600 Pro installed, since day 2 (under windows)
When that configuration I have gotten the following frame rates in games under XP Pro SP2 (all recent updates)
Half-Life 2: Lost Coast (1920x1200, full HDR, max settings) 74.8fps avg
Counter Strike: Source (1920x1200, full HDR, max settings) 97.3fps avg
Doom 3 with FPS cap removed (1920x1200, ULTRA HIGH settings, 2x AA) 116fps avg, 88fps min
Unreal Tournament 2004 v3369 (1920x1200, max settings) 71fps avg
Americas Army v2.8.2 (1920x1200, max settings) 66fps avg
Enemy Territory: Quake Wars (1440x900 max settings) 38-45fps
I recently installed the Catalyst 8.0 Beta driver and spiced things up a bit by trying the desktop HD 2600 XT driver.
In CS Source I now get 99.8fps with same settings
An amazing leap to 55-61fps in ET Quake Wars, same settings as well.
and about 35fps with spikes to the high 40's average in Bioschock demo at 1440x900 max settings, which I didn't test with the 7.9 drivers so I don't have anything to compare to. I didn't get FPS numbers but Company of Heroes runs amazing, with everything maxed out at 1920x1200.
Stock clocks on this card are 600Mhz Core and 685Mhz Memory if nobody has posted that yet. Also you can not overclock this card that I know of, I've tried every solution I used on my gaming PC with a X1800. ATI Tool won't work, either will ATI Tray Tool. And of course there is no OC settings in Catalyst Control Center.