Hi tom,for that kind of budget i think you will struggle to get some of the items listed above.....a gtx 780 would take the majority of that budget,let alone adding the latest i7 + ssd's.I think you should be looking towards at least a i5 3570k or if you could go the AMD route which are very reasonably priced,such as the FX 6300 six core £88.36 with scan.Memory you would probably get a good set of 8Gb for around £50 either gskill or corsair and i would go with a 128Gb ssd and use any existing hard drives and optical drives you are using for the moment,and as mentioned above dont scrimp on the power supply ,get a good named brand with plenty of headroom in case you upgrade other things later....
At the end of the day its down to personal preference at what you are willing to spend,but if someone gave me £900 and said build me a pc,i would probably put together something along these lines.....
mobo..
http://www.scan.co.uk/products/asrock-970-extreme4-amd-970-s-am3plus-ddr3-2100mhz(oc)-sata-iii-6gb-s-sata-raid-pcie-20-(x16)-atx-us
cpu..
http://www.scan.co.uk/products/amd-fx-6300-black-edition-vishera-six-core-s-am3plus-35ghz-14mb-total-cache-95w-retail
ram..
http://www.scan.co.uk/products/4gb-(2x2gb)-corsair-ddr3-vengeance-jet-black-pc3-16000-(2000)-cas-10-10-10-27-xmp-150v
psu..
http://www.amazon.co.uk/XFX-P1-850S-NLB9-PRO850W-Edition-Supply/dp/B0045L4BNW/ref=sr_1_3?ie=UTF8&qid=1372157716&sr=8-3&keywords=xfx+psu
case..
http://www.scan.co.uk/products/cooler-master-haf-912-plus-black-mid-tower-performance-gaming-case-w-o-psu
ssd..
http://imperialprice.com/index.php?route=product/product&product_id=520998
hdd..
http://www.amazon.co.uk/Seagate-ST31000524AS-Barracuda-7200rpm-Buffer/dp/B004HBAGSO/ref=sr_1_2?ie=UTF8&qid=1372159229&sr=8-2&keywords=hdd+1tb
cooler..
http://www.amazon.co.uk/Cooler-Master-Hyper-212-120mm/dp/B0068OI7T8/ref=pd_sim_sbs_computers_6
gpu..
http://www.scan.co.uk/products/2gb-evga-gtx-770-superclocked-acx-28nm-pcie-30-(x16)-7010mhz-gddr5-gpu-1111mhz-boost-1163mhz-cores-1
optical drive..
http://www.amazon.co.uk/Sony-AD-5280S-0B-Protected-Enclosure-Emergency/dp/B005DX4DAM/ref=sr_1_6?s=computers&ie=UTF8&qid=1372159646&sr=1-6&keywords=optical+drive
This lot comes in at £894.98,so just under the £900 upper limit you qouted......these are just a rough guide as to what you could get and what would be easily capable of playing the latest games.As i said its all down to what you want and your budget but do your research and decide which route you want to take be it either AMD or Intel...