Does it have to be wired? I have a wireless with the charge cable so I can plug it into my computer's USB port. I'm hoping there's a way to use this controller with Steam, otherwise I have to buy a wired controller? Once I plug it in I shouldn't have to do anything, isn't that right?

I have no experience with Steam but the wireless + charger cable works for me for any other game that supports it.
Don't forget the official microsoft driver thingie.

Yes it has to be wired, I happened to have one, Assassin's Creed is much more enjoyable with a controller btw.  I have Windows 7 and it found it when I plugged it in, but I still had to update the drivers before it recognized it.  All in all it's totally worth it

" Nope, you'll have to have an actual wired conrtoller.  The charge kit only charges the battery pack, it doesn't actually transfer the controller's actions to anything.  Or you could also get one of Microsoft's wireless controllers made for PCs  http://www.amazon.com/Microsoft-Xbox-Wireless-Controller-Windows/dp/accessories/B000KA5T6A  Personally, I like the wired controller since it's easier to go back and forth between the consoles and you don't have to worry about charging or replacing batteries.  "

The play+charge kit w/wireless works, nothing more is needed. 
 
Edit: Actually, never tried that with a steam game. Will try it out myself.

You might need to get the wireless adapter dongle thing, but first try it out with the charger cable. Let us know how it goes :)

For Steam it has to be wired, go to Fry's and buy a wired 360 controller. 
 
Just remember it only has to be wired, I remember I was at like Best Buy and it had the wired controllers next to the official Microsoft 360 pads for PC controllers and they were like 10 dollars more expensive.

You either need a wired controller, or an Microsoft's wireless Xbox receiver for windows (this thing). The Play & Charge kit won't work, all that cable does is charge the battery pack - no data about your button presses is being transmitted via the wire. All that data is still going via the wireless connection, and you'll need the wireless receiver for that data to be received by a PC.

Does anyone know a way to control the Steam UI with a controller? I'm trying to set up a PC in my living room as a console, and being able to launch a game via a controller would really help.

^No idea, I'd like to see the Xbox Guide button being used instead of doing nothing really but powering the controller on. 
 
Its funny how I ended up from using a 360, jumping on the PC and buying a controller for that after being fed up with the funky movements on keyboards...

Don't know if this is cheaper, but my cousin got me this for Christmas.  Works great.  Man, so glad I got this just before I started playing Prince of Persia.  Comp controls would have been ridiculous!!  Now I can also buy racing games!
 
EDIT:  To be specific, I actually own this one.  Cheaper cuz of no rumble...

" You either need a wired controller, or an Microsoft's wireless Xbox receiver for windows (this thing). The Play & Charge kit won't work, all that cable does is charge the battery pack - no data about your button presses is being transmitted via the wire. All that data is still going via the wireless connection, and you'll need the wireless receiver for that data to be received by a PC. "