Modding #1: Graphics Card

Original: ATI Radeon HD 2600 XT with 256 MB RAM in double-wide 16-lane PCIe slot
This card is rather slow, but noisy at the same time.

Replacement: ATI Radeon HD 5770 Mac Edition
  Chipset Model:    ATI Radeon HD 5770
  Type:    GPU
  Bus:    PCIe
  Slot:    Slot-1
  PCIe Lane Width:    x16
  VRAM (Total):    1024 MB
  Vendor:    ATI (0x1002)
  Device ID:    0x68b8
  Revision ID:    0x0000
  ROM Revision:    113-C0160C-155
  EFI Driver Version:    01.00.436
  Displays:
S2231W:
  Resolution:    1680 x 1050
  Pixel Depth:    32-Bit Color (ARGB8888)
  Display Serial Number:    27540018
  Main Display:    Yes
  Mirror:    Off
  Online:    Yes
  Rotation:    Supported


http://www.amazon.com/Apple-ATI-Radeon-5770-MC742ZM/dp/B003Z6QH6M

(I purchased it at Apple Store, where it is no longer available)

Upgrade Process: Plug'n'Play. The only risk I took here was that this card was not offered as an official upgrade for my MacPro Early 2008. I swiftly found several confirmations that my MacPro will work with this card. And it was easy, worked out of the box and survived every single OSX upgrade since then. So this upgrade was a no-brainer.

Benefit: Hard to tell, since I am no gamer. It was probably the right decision to make this rather massive upgrade on the GFX side.