We have a 1920's house with some 1/2" X 6" planks. Unfortunately the floor is in quite rough shape with finger wide gaps in some spots, I started to attempt to sand and flatten it but I think it will be too much work. That said, would the proper course of action be to put down some 1/2" subflooring (particle board or plywood tongue and groove) first. Screw it down then lay the laminate over top? If not what would be the correct thing to
If your subfloor is only a 1/2 inch thick and partially rotten you need to replace it. Even if it were in perfect condition that's not thick enough for laminate flooring, it will droop and bounce like walking on sponge cake. Cut out your floor boards, true up/replace/sister your floor joists using a site laser or string line and power plane if they need it (and I'd bet you a shiny nickel they do)especially if the joists are under the span load limit, here's a tutorial on span limits: http://www.awc.org/technical/spantables/tutorial.php It looks daunting but its not as bad as it looks. If you're still fuzzy just go to your local building center and they can usually calculate loads and spans for you. Once you've got that figured out, sheet with 3/4 t&g cdx. Run a bead of PL on the joists to fill and small voids and bind it all together. Here's a pretty thorough link from a sheeting manufacturer. http://buildgp.com/wood-products-installation-tips Hope this helps!
As long as your floor is "flat" then you really need to follow the install advice for the type of laminate you are installing. Some laminates require nothing, some a pad.
If your floor has a lot of damage and bumps then you probably want to patch it with a compound like floor leveler. You don't want to have large potmarks (anything greater than an inch) in your floor and leave it thinking the pad and laminate will cover it.
Given what you have, I'm going to beseech you - STOP sanding, you're only making too-thin flooring even thinner. If you keep going, you may fall through.
Were it my house or project, I'd lay 3/4" tongue-and-groove OSB (Oriented-Strand Board) down over the existing planks, NAILING it down with ring-shank nails (no screws). It may take some doing to select the correct-weight hammer. After the OSB is down, then lay your finish flooring on it.
For most things, I like to avoid OSB. For this, though, it's almost the perfect material.