2560 x 1440 on 2600XT?

I realise the Apple 27" display is ruled out due to lack of DisplayPort on that card.

There is an adapter available:
<http://www.atlona.com/Atlona-Dual-Link-DVI-to-Mini-DisplayPort-Converter-USB-Po wered.html>
but it won't help if the card doesn't support the resolution. In any case, you could replace your card with a much better one for not much more than the cost of the adapter:
<http://store.apple.com/us/product/MC742ZM/A?fnode=MTY1NDA5OQ&mco=MTg2OTU0NzM>

My system:


Chipset Model: *+ATI Radeon HD 2600+*
Type: GPU
Bus: PCIe
Slot: Slot-1
PCIe Lane Width: x16
VRAM (Total): 256 MB
Vendor: ATI (0x1002)
Device ID: 0x9588
Revision ID: 0x0000
ROM Revision: 113-B1480A-252
EFI Driver Version: 01.00.252
Displays:
Display:
*+Resolution: 2560 x 1600 @ 60 Hz+*
Pixel Depth: 32-Bit Color (ARGB8888)
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported
DELL E248WFP:
Resolution: 1200 x 1920 @ 60 Hz
Pixel Depth: 32-Bit Color (ARGB8888)
Mirror: Off
Online: Yes
Rotation: 90

I don't see 2560x1440 listed as a resolution my 30" monitor supports, but I don't know why the vid card wouldn't since it easily supports the higher resolution.

The ATI Radeon HD 2600 was designed before the 2560x1440 display, so it may not support that resolution, even though it does support higher resolutions. If it doesn't it may be possible to force the desired resolution with SwitchResX
<http://www.madrau.com/indexSRX4.html>
or DisplayConfigX
<http://www.3dexpress.de/>