So first, nothing works out of the box... there is always some software that needs to be installed and some configuration that will need to be done to optimize things for your specific setup.
Second, I would say you should provide as much power as you can. Mining is a very power intensive endeavor and making hardware run at it's limits will shorten its life. From what I understand you can get away with running only the 8 pin if you are a gamer but I would personally do everything I could to provide as much power as possible. The power draw is going to be what it's going to be regardless of how many pins you provide. Creating a power bottle neck at the connector is just going to result in inefficiency. If you are mining you are trying to make money, give the boards everything they need to perform.
Third, it would be helpful to know some more details. A week ago I would not have thought about them but after researching I have found these things to be important...
Are you using an HDD? If so, you should not even plan to use it and get a SSD instead. I have also found that when installing the OS, it is helpful to set up a 4GB swap partition as well.
How much RAM do you have. The more the better (as is usually the case). When you sync the Blockchain, having more RAM will let you load a bigger cache size. I am currently syncing with a cache=1024 which is common on the forums but I have recently seen people going as high as 2048 with 8GB of RAM. I have 4GB of RAM so I feel like 2048 is too agressive but I could be wrong and would encourage comments.
If your are planning to run Linux, make sure you don't mess around with the open source drivers and go straight to the AMDPRO-GPU drivers. It looks like your hardware is supported. Depending on your OS you can find it here.
IMO I would not mess around with the web wallets or Parity in particular. There are some recent developments with Parity that are concerning and many of the forum posts drive people seeking info in that direction. PLEASE read this article to learn more and also this one. The consensus is that the Ethereum Wallet and Mist are the most secure and work the best. The major problem with these tools is the INCREDIBLY long sync time. It seems that the initial sync can take days at this point with the highest block somewhere around ~46700000.
After messing around with the Ethereum Wallet GUI I decided that it was too frustrating and did not provide enough feedback on sync status to know that it did not hang. I started using the command line in Linux and using Geth.
In Ubuntu (or other Linux distros) the command looks like this to get the Blockchain sync started once everything is installed:
geth --rpc --syncmode=fast --cache=1024
The default since geth 1.7.3 is to run fast so you could also just use
geth --rpc --cache=1024
It seems like you are doing everything on the same computer so I added the --rpc flag. Like I said before, if you have enough RAM, you can bump the cache up to 2048.
If you are running Linux I would also open a second terminal and run the following command to provide a meaningful continuous sync progress update:
while (( $(geth --exec "eth.syncing.highestBlock-eth.syncing.currentBlock" attach) > 0)); do echo $(geth --exec "100 * eth.syncing.currentBlock / eth.syncing.highestBlock" attach) % Complete; done
While the sync is going on you can benchmark your GPU performance. I like Etheminer and the following command will allow you to make sure everthing is running right. If you are running Ubuntu you can check that your hardware is being found by first running (you may need to install it first)
sudo apt-get install clinfo
then run Ethminer with the following flags to check your hash rate:
ethminer -G -M
The -G flag is used because you are running OpenCL with your AMD hardware and the -M is the benchmarking flag. Be aware that according to the folks at Ethminer, as of version 0.9.3 the benchmark tool is broken for OpenCL and it throws an error after the 5th trial. This is not a big deal as it does give you a good estimate of the hash rate you can expect. If you want to look at individual cards you can use:
ethminer -G -M --opencl-device X
Where X is the Device ID according to the output of clinfo.
Be Aware: Syncing the full ethereum block chain will take several days depending on your system and the speed of your connection. Also, if you stop before the initial sync is finished, it will need to start over. Once it's complete you can run this command to start mining with all your GPUs (make sure you keep geth running):
Good Luck and happy mining
PS... if anyone is using Legacy hardware like the AMD Tahiti HD 7970/R9 280 or any other Southern Islands HD 7900 series GCN 1.0 hardware, I have a written a comprehensive guide on how to get you going.