Okay my issue is that I would like to upgrade for Battlefield 3 and my current set up is even unable to run BC2 properly (I have to run it windowed and at low settings).
I hope to achieve 1080x1920 res and more than medium settings but I am not too bothered about MSAA. I will only be upgrading my GPU and PSU and my budget is under £100. I have looked at Radeon HD 5770 1GB but I don't know whether this will be capable.
Current set up -
AMD Phenom X2 545 @ 3.0ghz
4GB DDR3 RAM
21.5" HP monitor
Windows 7 64 Bit Home Edition
ATI Radeon HD 4650 1GB
(I will be upgrading PSU according to the choice of GPU)
Your budget does not help nor does bloated and excessive UK prices :s
I suggest that you upgrade your psu first and try to find more money for later when it comes to the card. I am not so sure about the second hand market in the UK either but it is worth a shot if you can get something in good condition at a fair prices.
For your investment of £150 you will find little change in the capability of your system. You will likely have to invest more in order to get the change you desire. Save up more cash and hope prices drop.
Antec is also good but go with something that is worth your time. Choosing from a god brand is the first step then it is rating and quality is the next. Get a 80+ Bronze or Silver rated 500W or better psu. Those types of units are very reliable and come with energy saving features built in those reduce how much power is wasted during normal use.