Portal 2 doesn't really require much graphics horsepower at all. Source is quite dated and, while it can look quite nice, was originally designed to run on cards as pathetic as the Radeon 9600 (almost ten years old now). I was playing Portal 2 at full shininess on my GTX260, which is a last-generation budget card. Anything that you can still buy with actual money will run Portal 2 just fine.
I guess the point here is twofold:
--You don't need to spend much at all to be able to play Portal 2.
--DON'T make the mistake of using Portal 2 as the target for a new PC's performance. That's like building a car to do a 19 second quarter mile. You're going to wish it could do better the minute you're finished with it.
Some gpus don't require any extra power outside of the pci-e slot it is going to be plugged into. This makes installation less annoying and if messed up could damage the card/pc/comp. Most importantly cards that don't require outside power will allow you to use low end power supplies to run the cards. It sucks spending $150 dollars on a card then realizing you need a new powersupply (PS) to boot. PS are a pain because it requires a whole rewiring of your pc to install it.
This is generally only true of very low-end entry level cards. These cards are based on the lower-end tech but underclocked, undervolted, and make less heat which means they require less power to cool. All of that adds up to a card that can be powered off the PCIe bus itself. However, any card worth actually spending money on new-in-box will require AT LEAST one auxiliary power input. My GTX260 and my Radeon 6870 both require TWO aux power inputs.