Nvidia has its Gameworks libraries which includes faceworks, waterworks, fireworks, hairworks, cloth simulation,flex and so many other physX features as well as technologies like HBAO+ and TXAA temporal antialiasing. They're implemented in Unreal Engine 4 so many developers will be able to take advantage of them. AMD does have Mantle API and Tressfx(hair simulation) and both Cryengine and Frostbyte 3 are implementing mantle. So far the advantages of mantle have only been in performance and tressfx has only been implemented in Tomb Raider. DirectX 12 supposedly will offer similar low level API but we should most definitely see some very noticeable visual improvements and features. Performance advantages can go either way although AMD cards do offer better price to performance ratio. Multi GPU setup is preferable as games are starting to become very demanding as we're getting into real "Next-Gen". Watch Dogs for example requires a GTX 780 and an i7 4770K to run at Ultra settings and The Witcher 3 although still too far for its release humbles the GTX 780ti with only 35-45 FPS at maximum detail. Most definitely upcoming games like Assasin's Creed Unity and Batman Arkham Knight which are both developed exclusively for next gen will be equally demanding too. A single high end GPU for now will be appropriate and give the opportunity for a second one down the road. A good price for an AMD R9 290 or 290X is a good option but so is a GTX 780 or 780ti (a good condition used one).