*waits for JC*
Hey I was on holiday n' stuff

I'm not overly familiar with Dayz personally, but in general Patty's suggestion is pretty good - it would give you a nice upgrade without having a ridiculously overspec'ed card relative to the rest of the system. It's probably at or slightly above the 'recommended' spec in the screenshots provided.
Those 750/750 Ti's are amazingly efficient for the performance they supply, so you can normally get them without needing additional power connections. Be careful with which one of them you get as I wouldn't guarantee that your current Power Supply would have a PCI-E power connector (which some 750 Ti's require, it depends what their specs are).
Here's a link to that card in the UK:
http://www.scan.co.uk/products/2gb-evga-gtx-750ti-pcie-30-5400mhz-gddr5-gpu-1020mhz-boost-1085mhz-cores-640-dport-dvi-hdmiNote - before anyone goes 'AMD is better for that price' - yep, and they have drivers that make the Russian govt look reliable so I generally avoid them at all costs, they're also a lot less power efficient at that price point.
The only thing I would be worried about is the age of your computer - the i5-650 is getting pretty old now, so if you've got a ~300w ish power supply it might be worthwhile replacing it, if you've got anything less than 300w then I would say it definitely needs changing (the total watts supplied should be clearly printed on your PSU). That could potentially add another 75 pounds ish for a decent one.