SLI (Nvidia) or Crossfire (AMD) are technologies that allow you to use multiple videocards at the same time, increasing performance and raw data throughput.
Performance doesn't increase linearly though as adding extra hardware adds communication and processing overhead, but graphics usually scale well because they reduce to matrix algebra.
I consider it to be a premium solution to performance, although sometimes it can be cheaper than a premium single card. In any case, this is for scenarios such as 4K resolutions and so on. Single monitor setups at 1440 or less are just fine with one 970.
I see. Well, I'm not 100% sure as to whether I would use them in the future as if they don't make a massive difference I can probably do without.
The updated version of my desktop PC is now this, please let me know if I've missed anything!:
Processor (CPU): Intel® Core™i5 Quad Core Processor i5-4690K (3.5GHz) 6MB Cache
Motherboard: ASUS® H97M-E: Micro-ATX, LG1150, USB 3.0, SATA 6GBs
Memory (RAM): 16GB KINGSTON DUAL-DDR3 1600MHz (2 x 8GB ) -- I know people told me that 8GB is plenty so I don't need 16GB but there wasn't an option to get 2 x 4GB so I decided to go with 2 x 8 instead. Because the price isn't too much higher and I'm guessing it's better than having 1 x 8, right?
Graphics Card: 4GB NVIDIA GEFORCE GTX 970 - 1 DVI, 1 mHDMI, 3 mDP - 3D Vision Ready
1st Hard Disk: 500GB 3.5" SATA-III 6GB/s HDD 7200RPM 16MB CACHE
1st DVD/BLU-RAY Drive: 24x DUAL LAYER DVD WRITER ±R/±RW/RAM (I know I could do without this but if I don't have this, does it mean I don't have a CD-drive in my computer? This is the cheapest option besides not having one... but if I still have a CD-drive in the PC without this then I am happy scrapping it)
Power Supply: CORSAIR 650W CS SERIES™ MODULAR 80 PLUS® GOLD, ULTRA QUIET
Memory Card Reader: INTERNAL 52 IN 1 CARD READER (XD, MS, CF, SD, etc) + 1 x USB 2.0 PORT
Processor Cooling: CoolerMaster Hyper 212 EVO (120mm) Fan CPU Cooler
Extra Case Fans: 1 x 12CM Black Case Fan (configured to extract from rear/roof)
Sound Card: ONBOARD 6 CHANNEL (5.1) HIGH DEF AUDIO (AS STANDARD)
Operating System: Genuine Windows 7 Home Premium 64 Bit w/SP1 - inc DVD & Licence
Keyboard & Mouse: CM Storm Devastator Keyboard and Mouse
Altogether the cost is (incl VAT) £1003 which is amazing!
And regarding using Linux, I would be tempted but the support for Linux in gaming doesn't seem to be as good for Windows. Also, erm, I have family who work at Microsoft so they'd probably murder me if I abandoned Windows.