Best graphics card for £50

PNY GT 520 PCI-E Graphics Card - 1GB http://www.pcworld.co.uk/gbuk/pny-gt-520-pci-e-graphics...

Novatech GeForce GT 630 2048MB GDDR3 http://www.novatech.co.uk/products/components/nvidiagef...

ASUS GeForce GT 440 1024MB GDDR3 http://www.novatech.co.uk/products/components/nvidiagef...

XFX AMD Radeon HD 6670 1024MB GDDR3 http://www.novatech.co.uk/products/components/atiradeon...

is this one any good

http://www.amazon.co.uk/Asus-Graphics-PCI-Express-Cross...

That last one doesn't appear to have a fan. For general or HTPC use, it would be great. For gaming, it might overheat unless you have really good airflow through your case.
The HD6670 is the strongest of the cards you've listed (in some cases by a lot), but if you can, get a GDDR5 version rather than DDR3.

you can try getting a hd 5670 gddr5 for 40-50 pounds...

ok this one then? will it play arma 2?
http://www.amazon.co.uk/Asus-Graphics-PCI-Express-Cross...

If you want to play ARMA2 without any hiccupts or problems I recommend a used 5770 from ebay, wont cost u 50 quid for a decent deal. ELse u can add around £20 and get a HD 7770. In my opinion itis worth the extra 20quid on top.

CUrrently a HD 7770 fetches for around £80-90, but you should tell u more about your system before upgrading just incase there are other issues.

If you have a weak PSU, the strongest card you can get is a HD7750.

HD7750 > HD6670 (GDDR5) > HD6670 (DDR3) >= HD5670 (GDDR5) > HD5670 (DDR3)

Which is your power supply? If you post your PSU, then we can recommend you a card according to your wattage capacity.

Mind that your CPU may bottleneck the new card, so post your config, please (cpu, ram, psu, etc).