I bought my GPU second hand off ebay.
I may have spoken too soon as my system just froze with nouveau drivers installed.
I personally believe that the video memory etc, may be faulty.
I suppose the best thing for me to do, is buy an Nvidia gpu for approx £20, and try to install Nvidia drivers.
Then afterwards if no joy try another monitor.
As i have read about issues with the edid file
Asus p5ql-e * Intel e8400 core 2 duo 3.0 ghz * 4 gb ram * Palit Geforce GTX 460 1gb * Realtek alc1200 8 - channel h/d audio codec * DVD/RW - Samsung atapi ihas124 * HDD Seagate st3320613as 320 gb * Linux Mint 15 Olivia Cinnamon 32 bit