I registered on the forum just to add my view to this thread as I've pretty much gone through the OP's thought process and wanted to recount my experience.
I, like most people who live in the real world, can't afford to just throw money at building the latest and greatest rig. I own a mid range PC built mainly from bargains I've sourced on Ebay plus bits I've added to improve it here and there. I run an AMD Phenom II X4 965 with 8Gb G.Skill memory and an ASUS 560 Ti - not great shakes I know but hey, we're not all millionaries Rodders (Delboy pun for the UK boys amongst us).
Anyway, I've just upgraded from a Gigabyte mobo to an ASUS Sabertooth 990FX which supports SLI - bargain Ebay price £90. I've also added a second ASUS 560 Ti for £65.
The reason I went down this route is having played BF3 only high/ultra settings before, I was sorely disappointed to see that geforce experience set everything to LOW or OFF on BF4 with one card- not pretty I can tell you.
However having done the 'El Cheapo' upgrade to 560 Ti's in SLI, geforce experience is now cranking most things to ultra and running HBAO to boot, all at 1920 x 1200. So for not much money, and SERIOUSLY LESS than buying a new GTX 770 or AMD equivalent, I can play BF4 as it should be played with frame rates averaging in the 50/60's
Now you can argue the semantics of micro stuttering this, game profiles that and SLI/crossfire the other, but the reality is that buying a second mid range card can and will significantly improve gaming perfomance so long as you understand what it takes to run one.