its all about having a Gcard that offers you the option of attaching 2 monitors, near all modern Gcards will allow this there is nothing special about having this feature on a Gcard, only using onboard graphics could ever possibly restrict you to a single monitor, only real thing to be aware of if your choosing a Gcard to set up duel monitors, is what outputs you have on the card, ideally you want to know before had what inputs your going to have on your monitors, say for example both your monitors are DVI, then get a card with 2x DVI out ports, there is a large selection out there from as little as £20 to £800 depending what specs your wanting, so no point converting ports from DVI to HDMI, just pointless too much selection to get what you need straight up at all price ranges.
I know a friend who has 3 monitors set up, 2 via his Gcard and and then a third one via the onboard chip in the CPU, this is a real pain setting it up in this way though, hard to even explain the process of getting 2 Gcards of complete different manufacture without using cross over or SLI, the roots are in the set up process from installation, you have to do everything in a very specific order which I dont 100% recall, its like install install windows with Gcard attached with onboard disabled in the Bios, then change the Bios to disable the Gcard and turn on the onboard taking you back to safe mode to get the drivers installed for the Intel chip on board, then disabling them again moving back to the Gcard in the Bios before finally enabling the both together for both driver sets to work together, but thats not really the best way its just a cheap bodge way to get a third monitor working as appose to having the right hardware and using say 2x DVI + 1x HDMI from a single capable card, or running cross over or SLI giving you all the ports you need from a single set of drivers