Welcome to watercooling!
First off, what case do you have? Do you want to mount all of the rads internally? If so, you will need a case where you can mount radiators, ideally natively.
Now to build a loop around your graphics cards, you will need:
Pump
Reservoir
Tubing
Fittings
GPU Blocks (of course)
Radiators
Fans
The first I would do is try to get an accurate idea of how much radiator you need to dissipate the heat from those GPUs. If memory serves the 7970 has a TDP of ~215 Watts. So 430 Watts is what a pair of 7970s would be consuming at the absolute maximum. Now realistically, the cards will probably only consume about 400 Watts at full load maybe less.
Now that you know this you can select a radiator.
There is really good chart for estimating the heat dissipation of a radiator:
http://skinneelabs.com/xspc-rx360-v2/2/
The center top chart (pink and blue) is for an XSPC RX360 Radiator (3 x 120mm). From this chart you can look at how much heat you have to dissipate and from this you can get an idea of how high you will have to run the fans attached to the radiator. It also gives you the expected temperature delta. This is really important. It is simply the difference between the air temperature in your room and the temperature of the water in your loop. Obviously, the lower the better. A delta of about 10 C is considered average. If you can get it to about 6-7 C, that would be nice.
Given the heat load of 400 Watts, I would recommend nothing less than a 360 Rad like the RX360 (you might be able to get by with a 240 Rad RX240 but you will have to run the fans @ 1800 rpm or so). You will be able to get rid of a lot heat and still be able to run the fans very low, keep your system quiet and maintain a good temperature delta. If size constraints are an issue you could use an equivalent such as an RX240 + RX120. Now you don't have to use rads from XSPC. There are a lot of other brands but you need to keep in mind the rads need to be fairly thick to dissipate a lot of heat. Not all rads are created equally. But XSPC rads are good for their performance and for the fact that they are cost effective.
By the way, why will you not be putting your CPU into your loop? I am just curious why you wont be as the CPU would add significantly more heat to the loop.
At this point, you just have to decide if you want to go custom or with a kit. The advantage of kits is that all of the parts that you need are included such as fittings, tubing, pump etc and cpu block. They are also cost effective. Kits like the XSPC Raystorm Kit or the EK H30 are good kits. You can find kits at dazmode.com. He has lots of options for kits. He is in Canada though. I would say though if you are going to add the CPU to the loop you should think about going custom because the restriction of water flow of 3 blocks may be a little much for a pump typically included in a kit.
Now with custom you can select your reservoir, pump, tubing, fittings etc.
Reservoirs are typically selected based on preference. Bay reservoirs tend to be a bit more difficult to bleed to get the air out of the loop but they look cool IMO. Tube reservoirs are nice though because they are easy to bleed and don't use up front expansion bays. As for the pump, you would want to select one that can produce strong flow to overcome the restriction of the blocks to keep the water flowing nicely. I typically recommend a Laing D5 (also known as MCP 655). You could go with a DDC pump though. Tubing is again a preference thing as the diameter of the tube really doesn't matter much, only for aesthetics. I would recommend compression fittings over barb fittings though because they are more secure and you can disassemble the loop if needed very easily.
So I would just do some research and see what is out there. This is your loop so I am not going to recommend certain blocks based on what I think looks cool. I can say though that for GPU blocks, there is literally no difference in thermal performance between the GPU blocks from different companies.
Rubix_1011 has a really good sticky that talks about building water cooling loops in much more detail. This should help you figure out what parts you want to get.
Cheers