Adding 4850 with current 2600 - How to?

You need a 6 pin to 6 pin molex cable to connect to the card to the Mac motherboard, you can buy it from ATI if you live in US or from http://www.svideo.com/x1900.html if you live outside US.
This cable draws the power the card needs so you don't need to worry about how many hard disks etc you have.
The fan on the 4850 goes all the time at full blast when running in Mac OS X which may or may not annoy you.
The 2600 can give you a dual desktop with two monitors in Windows Vista which is nice if you have a second monitor. The ATI drivers recognise the 2600, if you run a non ATI there is some messing setting up which card to use from what other posters have said.

Not sure about how easy it is to add a 4870x2 do a search here and see about that.

Installing the card is straightforward. Getting it back out can be tricky as the plastic tabs that hold it in place can be tricky to open.

there is an earlier thread where someone posted a pic of the connector for the cable on the motherboard, there are two of them near the power supply.

 

Place it in Slot 2. Use a 6 pin to 6 pin PCIe cable. This will take care of the physical installation.

OS X would ignore it I think. On the windows side, you'd have to open Device Manager and disable the 2600 IIRC.

You can use it in Boot Camp, not sure about VM Fusion, as I don't run it.

This card won't kill the PSU, unless you have a massive power draw from something else not mentioned. It's rated for 980W.

For use in windows only, you can install other cards. The 4850 or the 4870X2 would work. Some have even installed an nVidia GX280. Your choice.

Hope this helps.

 

You don't need to worry about the power draw, that much I can tell you.

I've got the 2007 8-core 3.0Ghz model which is more power hungry, 2x10,000RPM Raptors, 2x7,200RPM Barracuda ES, all my DIMM bays are full, both of my optical bays are in use, and I have an 8800GT and an 8600GT installed.

 

Why the 8600GT?

 

PhysX board with the ForceWare drivers