Budget second PC

Currently run a second pc (my old first pc with a few mods) for gaming with my mates.   I can't stand how many games have completely dropped local co-op these days.   Anyway the motherboard has given up the ghost, I thought it was the PSU and replaced but it has not helped.   So I would like to build a new one, anyway I'll list details in accordance to NJM1112's guidelines.

I want to make my new pc good enough that it will be able to keep playing the games my number 1 pc plays, albeit at lower res and lower settings, until the day comes to upgrade my number 1 pc, which considering was only built in December 2012 will be some time. [Current PC specs listed on my profile]

Budget - basically the lower the better but let's put an upper limit of £300. 

I just recently bought a CXM500 which i would like to use as the power supply, I also have an ocz vertex 4 128gb in my second pc atm, I also have an old HDD lying around that I could use for this build.   I could easily sell the ocz to one of my friends if you think the money would be better spent elsewhere for this build.   The GPU i have in my old PC is a 4850 and works fine.

I also have an old case, however it is too small for modern GPUs and CPU coolers, it also has no dust filter and gets filfthy so I would quite like to upgrade the case.

PC sole use will be gaming.

Yes I can and will overclock however it is not an important factor i.e. better, less overclockable gear > worse, more overclockable gear. 

I do not want water cooling, and will not do any custom wcing or any custom work for that matter. 

I have an OS.

Obviously the better the settings I can play on the better, but I accept the latest games will not run on high settings, however it needs to be fairly future proof as I expect to be using my number 1 PC for a long while yet.

The monitor I will be using is 1680x1050 and I do not plan on upgrading it for some time.

Games I am likely to be playing in the near future will include Rome tw 2, Grid 2, BF4 when they come out..  

I am not bothered about motherboard features, as long as it has all the basic stuff.. only need 8GB ram, 1 PCI 2.0 16x etc

Currently I am thinking on spending a little more on the CPU and using my old 4850 which I could possibly upgrade in the future.. 

I am thinking of getting one of the AMD Piledrivers, but if you think there is a better intel one out there please suggest, would also like any suggestions with what motherboard, case and CPU cooler, and any ram.   So.. £300 for CPU, cooler, motherboard, case, and ram

Is it worth saving money possibly going wtih micro ATX build?

Did the conversion, and 300 quid is around $450.

This is roughly 330 pounds. Since this build has a 7850 in it, you can go ahead and sell that 4850. You can use a old hard drive and that SSD in this build to give it some storage.

8 gigs of RAM, FX-6300 with a 990FX motherboard, along with a Cooler Master 212 Evo. Very overclockable system with high quality brands.