Here's a quick way to get LOTS OF CAPS:
I have 60 000 caps, got it by much tedious waiting and trading of every damn thing I could pick up, repair or steal.
Invest in the traders... go to canterbury commons and talk to uncle roe, then spend the 2000 upgrading Lucky Harith, Crow and the other two wierdos... it'll be the best money you spend in the game.
Put lots of points into your repair skill early on. Then pick up every raider, talon, mutie weapon and armour you find and repair them so you can hold more of em in your inventory. Attack every enclave outpost you see because they have the most valuable goodies: A fully repaired Lazer rifle will net you 999 caps!
Warp to megaton and go outside, the traders come in two hour spells so just wait for them in hour long waits, (they are there from around 6am to 6pm) and sell all your ill gotten gains to them as they will have over 1000 caps each to spend which will replenish every 3 days.
Repeat this mind numbingly boring activity until you're rich... YAY!