Junk's rig should be able to play TF2 and L4D, I had an equivalent rig that ran those no problem, and I think the other games would work great on it as well.
You will need a hard drive to store stuff on. I would recommend on like this
Seagate HDD
You will also probably want a
dvd drive.
For the Hard drive, I would recommend one from Seagate or Western Digital. Other manufacturers drives will probably work, but I have had the best luck with those two.
As for DVD drives, pretty much any major brand should do you fine. I would recommend a DVD burner just in case you want to save any information on DVDs (like backups or pictures) but just a regular DVD reader will work for most gaming machines.
For your operating system, if you can get a copy for cheap from someone, probably go with that. Otherwise I would recommend getting a copy of xp, if you can find it. XP is still the best windows OS for gaming as it does practically nothing for you besides run the game.
If you feel like it, Windows 7 also is pretty speedy (not as quick as XP, but much better than vista, and just about as shiny as vista is), although you will have to look out for bugs until they get most of them fixed with the first service pack. I have been running it for a couple months on several computers and haven't noticed too many issues though. With windows 7, get the home version or the professional if you think you will need the extra features. The basic has nothing worth spending the money on and the ultimate's additional features probably won't be worth the price.
In any case, if you are going to buy a operating system, buy an OEM copy. There is no sense to paying an extra $100 or more for a pretty box.
As with purchasing anything, make sure to read the reveiws to see what other people have seen from the device.