As far as I know the 2600 doesn't have OpenCL support.  My late 2007 iMac has the same GPU and it's not supported.  I think, (not 100%) that the 4870 was the first OpenCL compat. GPU in the MacPro.  Look for a cheap GPU upgrade on eBay for that machine.
I put a 5870 in my 2008 MacPro, you can find them on eBay for a reasonable price.. Also as of 10.8.3 6xxx and 7xxx cards work with the proper power connectors.
This thread might help...

Oh, just want to add I'm damn impressed seeing the total comp power of the bitminter pool grow by over 1Thps in the 2 1/2 weeks I've been a member.  That's impressive growth.

Just wanted to say hi to the Bitminter Community.  I tinkered with BitCoins shortly after their release a few years back.  It was very new and frustrating.  I mined some coins over a few months, but the confusion never really cleared up so I abandoned it.  Now a few years later I'm back and after seeing the price for coins really bummed I let the ones I mined back then just "go".  But, remember back then there was little value to them, and hind-sight is 20/20.  Anyway.  Crunching away again using the super easy bitminter client and just having fun with it I guess.  While the wallet function makes a lot more sense now on the Bitcoin side.  I'm having issues figuring out what to do with the namecoins I'm earning...  I've set my donation to 50% to the bitminter host since I can't figure out anything about them. Is there a simple noob guide somewhere that explains Namecoins?
Running now on a 5870, a 680MX, two 5770's (occasionally) and a few other machines that I randomly turn on and off depending on if other people need them.  When all is fired up I'm just around 1 Ghps.