OK. Here's my advice. If you are sure that you will get more GPUs later then you need to prepare a PC that can handle several GPUs. I found that a PC with 4 x Radeon 280X to be a decent performer. Each of these cards makes between 700 and 800Kh/s but most of the time between 710 and 750Kh/s. So what you should go for is :
1. Motherboard with as many PCI-E slots as you can find for a decent price. Those PCI-E slots could be of any type from 1x to 16x, because you will have to use PCI-E extender cables anyway for multi-GPU configuration.
2. Cheapest CPU you can find
3. Not much RAM, 4GBs is enough (probably less is also OK)
4. Any HDD
5. Video cards - Radeon 280x, 7950, 7970. Make sure you buy a model with unlocked voltage, so that you can undervolt them. An example of what brand you should not buy is - Gigabyte. While XFX, Sapphire, ASUS are OK.
6. PSU - this is critical. I found that with a good 1KW PSU you can run 4x Radeon 280x but ONLY if you undervolt them (well this also depends on the brand of the cards, and also on each card itself). By default XFX Radeon 280X consumes about 280W (again this depends on the brand and the default GPU voltage), but with just a slight undervolt from 1.2 to 1.15V power consumption falls to 230W. So a good 1 KW PSU like Cooler Master V1000 or Fractal Design Platinum 1000 can power 4 of these cards. You should get a device for measuring the power consumption. That is for ensuring that you are not over-loading the PSU.
If you find the information useful you can donate me some DOGE.
Thanks for your input. Here is the rig configuration that I'm planning:
1. MoBo: ASRock 970 Extreme 4 (I can easily add 4 GPUs to this)
2. AMD Sempron 145 (The cheapest processor I could find in the market)
3. 4 GB 1600 MHz RAM
4. 16 GB USB flash drive (I'll be using Linux)
5. GPU: The R9 280x offering by MSI
6. PSU: Corsair AX 1200W
7. A couple of fans for air circulation
8. Sine wave inverter - for power surge protection and power backup
Total cost: roughly $1275
I've been following this as a guide:
http://www.cryptobadger.com/build-your-own-litecoin-mining-rig/Thanks for the information about under-volting. I'd read about it in the guide I mentioned above, but I didn't realize that it would have this much of an impact on the total power usage.
I'd send you some Doge if I had any, but I'm just starting off in the crypto-currency world. Perhaps when I have mined "much coin" :-)