First time build, a few questions

Hi all, I'm getting a bit tired with my current PC and I think its time for an upgrade. Ideally, I want to keep the price as low as possible but I still need a build that can play games like BF3 and Planetside 2 well. My budget is around £700.
So here is what I've decided upon:

CPU: Intel Core i5-3570K 3.4GHz
http://www.cclonline.com/product/73989/BX80637I53570K/C...

Motherboard: MSI Z77A-G43 Intel Z77
http://www.novatech.co.uk/products/components/motherboa...

RAM: GeIL Black Dragon 8GB 1600MHz DDR3
http://www.cclonline.com/product/86356/GB38GB1600C9QC/D...

GPU: Sapphire HD 7870 2GB OC
http://www.cclonline.com/product/100506/11199-19-20G/Gr...

Storage: Seagate Barracuda 1TB (will probably get an SSD later)
http://www.cclonline.com/product/72741/ST1000DM003/Hard...

PSU: Corsair CX600 Builder Series 600 Watt ATX
http://www.cclonline.com/product/96476/CP-9020048-UK/Po...

CPU Cooler: Cooler Master Hyper 212 EVO
http://www.cclonline.com/product/76740/RR-212E-16PK-R1/...

Case: Gigabyte Sumo Alpha OR Cooler Master HAF 912
http://www.cclonline.com/product/82073/GZ-SU11BA-CTB/Ca...
http://www.cclonline.com/product/92775/RC-912P-KKN1/Cas...

There are a few questions/concerns I have. Firstly, I'm not sure which case I should go with, of course the HAF 912 costs more, but is it worth it?

Also, are there any parts that need to be changed, due to compatibility issues or because of a bad brand. I've seen a few threads on here where people have had issues with a GA-Z77 D3H and HD 7850/7870 where the motherboard won't recognise the graphics card when its using PCIe 3.0, and sometimes it won't even recognise the card when its using PCIe 2.0.

Finally, maybe a bit of a novice question, but do I need any extra cables or parts for my build? If so, could you please state what they are.

Thanks all