What is your CPU?
CPU block: EK supremacy because it has stunning performance, high flow to give pump breathing room if you have a lot of components in your loop. Finally because it is the most universal block, fitting most Intel and AMD sockets.
Pump: A D5 pump because they are the best performing ones, though if you want to save cash EK 4.0 will be fine for 3 high flow blocks.
Reservoir: doesn't matter which one you get, a pump reservoir combo might save you a few quid but will limit mounting options purely due to size.
Radiator's: For 2 7970's and a CPU, i would recomend a thick 360mm radiator to go on top (at least 47mm, considering your case and what your cooling, itl be a waste not to take advantage of the clearance). If your planning to run these components pretty hot after overclocks, i would definitely recommend another radiator, maybe a decent 240mm at the bottom of the case next to your PSU. EK, XSPC or phobya are great rad's. Alternatively you can jsut get one big 1080 radiator from phobya and keep it externally. This will give you all the cooling you will ever need and more for a really good price.
Fittings: up to you. If your new to watercooling i think compression fitting's are a way to go, they look good and are easy to install. Bitspower are crazy expensive but look nice. I would say avoid monsoon, they may look awesome but people have a lot of trouble using them (i personally am going to use them next build because i love the look enough to take the risk).
Tubing: XSPC tubing matching the size of the compression fittings or slightly smaller than barbs if you decided not to use compression. so if you use 1/2 barbs, use 7/17 tubing. There isnt a performance difference between tubing size. Clear tubing stains with coloured fluids over time but its not a big deal, i change my loop tubing every maintenance (which for me is every 6-8 months). Coloured tubing is the other option.
Cooling fluid: Get a premixed for ease since it includes everything already. Anything from EK/Phobya or from mayhems pastel or x1 range.
Fans: SP120's or GT's
The cheapest thing you could do is just get the OCUK V12 kit with 4 extra fittings for your GPU's. An extra 240mm radiator will make a world of differecne too (which will need another 2 more fittings. This is superb performance and includes everything you need for just over £300. The only things i would change would be only for aesthetics eg. bay reservoir for tube, barbs for compression fittings, CPU block for supremacy HF plexi.
