Not long had to build a budget replacement pc for uni as my old one went the way of the do-do.
One of the components that was not completely dead was the old graphic card with is an underpowered NVidia GeForce GT 320.
The system specs are i5-2500k oc'ed to 3.60, 16GB adata 1600Mhz RAM and two HDD's both old 1TB disks. Oh and the PSU is a 600W corsair gamer series.
My budget right now is tight but after christmas i would be looking at spending around £150 possibly pushing it up to £200 depending on how many shifts i can pull at the local pub and supermarket.
For £155 you can get a Sapphire HD 7850 from amazon.co.uk: http://www.amazon.co.uk/Sapphire-11200-07-20G-Graphics-HD7850-GDDR5/dp/B008HV63NM/ref=sr_1_3?ie=UTF8&qid=1354450602&sr=8-3
Also, if you can find a bit more money, for £205.86 you can get an ASUS GTX660(non ti) DirectCU II: http://www.amazon.co.uk/Asus-DirectCU-Graphics-DisplayPort-Surround/dp/B0098VKIRM/ref=sr_1_12?s=computers&ie=UTF8&qid=1354450750&sr=1-12