Ah you didn't mention the requirement for DDR3 thus needing to replace all your RAM. Thats a bit different as I was asuming that you were attempting to keep everything except your motherboard.
There are only 2 socket 775 boards on the OCuk website - take your pick:-
http://www.overclockers.co.uk/productlist.php?groupid=701&catid=5&subid=1490
They both only have 2 DIMM slots so you will be needing 2 x 4Gb modules so will want to budget about 40 quid for that.
You're essentially spending 80 quid to stick with your same processor and you could be into a socket 1155 for about 25 quid more, but of course you'd need another CPU but just about any 1155 CPU will play Dayz very well. My son plays it and since it looks crap on his Core i5 and a Radeon 6970 I can't imagine that it's either CPU or gfx intensive. It's an rpg/mmo/fps for the minecraft generation as far as I can tell.