hi all,
i have imac/intel 24 aluminum:
  Model Name:	iMac
  Model Identifier:	iMac7,1
  Processor Name:	Intel Core 2 Duo
  Processor Speed:	2.4 GHz
  Number of Processors:	1
  Total Number of Cores:	2
  L2 Cache:	4 MB
  Memory:	4 GB
and the graphic card:
ATI Radeon HD 2600 Pro:

  Chipset Model:	ATI,RadeonHD2600
  Type:	GPU
  Bus:	PCIe
  PCIe Lane Width:	x16
  VRAM (Total):	256 MB
  Vendor:	ATI (0x1002)
  Device ID:	0x9583
  Revision ID:	0x0000
  ROM Revision:	113-B2250F-219
  EFI Driver Version:	01.00.219
  Resolution:	1920 x 1200
  Pixel Depth:	32-Bit Color (ARGB8888)
  Main Display:	Yes
  Mirror:	Off
  Online:	Yes
  Built-In:	Yes
have installed 12.04 LTS and everything work as i wished but after few hours work on ubuntu the graphics crash or can say poor sounds like missing driver!!

Do i have to install extra driver for graphics or edit system conf ..etc?