Someone in my last thread noted that 3" tubing with 1/8" wall would be very weak. As i have no experience in this (just saw simaler sizes used in some after market parts), i could use a bit of advice here.

The same person said 2" or smaller would be best. What about wall thickness? The welder i'm looking at can do up to 3/16" thick. Its the only one I can afford (under $500).

The bumper will be atached to the frame using two 3/16" "C" angles, wich will be reinforced to provide a mounting point for a winch, tow hooks, and pre-runner. The angles will bolt onto the frame rails using two 3/4" class 5 bolts.

2" would probably be stronger because there would be less leverage on a given part of attachment ie the weld....

What welder are you looking at getting? I'm guessing it's a 110v and can do 3/16" with Fluxcore wire. Is that right? If so that's pushing the welder and the integrity of the welds.Especially if you are going to be making a winch mount bumper.
Regarding after market add-ons: if what you were looking at was 3" then it probably is considered a "show" piece ie. lightbars that mount in the back of pick-ups that look like rollbars, safari bars that mount on the bumper. Check out products that actually mount to the frame and come from a real 4x shop. PepBoys and such don't really carry anything trail ready.
If you are starting to weld check out this site:
http://www.hobartwelders.com/mboard/index.php?

unabonked, the standard tube size, at least in this part of the country, is 1.5" with .120 wall. To a lesser extent, 1.75" .120 is used. I have both on my truck.
I'm no expert, but there are plenty of them here in SoCal, and I figure they know what works best. :)

Yeah, i forget the make of the welder, but thats basically the specs capabilities. I origonally thought that 3" tubing would be best, but i have been told otherwise. I will probably go with 1.25-1.5" tubing with a .125 wall.

What difficulties will I be experiencing with using the this welder? will the joints be too narrow?

I'm using 1.750"x.120" tube on just about everything on my truck.

3" tubing is stronger than 2" tubing, because of the moment of inertia. I did not look it up but a 3" 1/8" tube would be as strong or stronger than 2" 1/16" tube. Yes I am an engineer.