I have my primary monitor connected via DVI and have two HDMI and two Playport connections remaining/ Which should I connect my secondary monitor to?
Without knowing what versions/types of HDMI (version number), DVI (single-link, dual-link), Displayport (version) you are connecting i would go for Displayport.
It's more what it's connecting too, but fuus is probably correct when he/she says it won't mater much. It's just things like HDMI 1.4 maxes out at 3,820x2,160 30fps, or a DVI single-link is limited to 1,920x1,200, etc, etc.
Your dvi is dual link so you can connect pretty much anything to it if you have a dual link cable, dont connect more than a 1080p (16:9) monitor to HDMI, high res to display port. General rules should be all you need
OK so why does it now show the secondary monitor (HDMI) as number 1 and the primary monitor (DVI) as number 2?
From what I gather those numbers mean very little as long as your primary display is set correctly in the Nvidia control panel. Look on the 'setup multiple displays' tab