ZachyBeat :
You can run multiple cards in the same computer independently but you cannot SLI two different cards. I'm not sure if your friend is being honest with you. He might have them both in his computer but not setup for SLI. Nvidia requires the same gpu and vram for SLI to work. For example you would need two gtx 970s that are 4gb. One with 2gb wouldn't work (I don't think that card exists but this is just as an example).
What resolution and games are you planning on playing? 60hz or 144hz monitor? If you're going for 1080p 60hz I would just grab a 970/r9 390. If you want to spend the extra cash and have it last you an extra year then feel free to grab the 980.
Thanks for the answer. The monitor I will be getting is the Acer KA220HQbid (on Amazon:http://www.amazon.co.uk/Acer-KA220HQbid-21-5-Black-Full/dp/B00Z03HSP2) as my current monitor is d-sub which I will still keep. I will still most likely get the 980 just for the future proofing unless the new 8GB cards are announced to be sold soon and then if the price of the 4GB 900's drop I might sli 970's or even 980's. Also i currently ave 16GB of RAM and a 4.2 GHz possessor so I want to make a my good rig great (as currently I am am running a k2000 XD)
edit: just looked at my motherboard it does not have 2 pcix16 it only has a pci and pcix1 so if i wanted to sli i would have to buy a new motherboard too...shame :{