First, most house holds own a computer. This computer on average probably cost $500. When Xbox 360 came out it cost $400. Now if you bought both of these items you spent $900. Why not put that $400 into that PC and now you have a $900 gaming PC. $900 will get you a great gaming PC. Maybe not the greatest but something that will play everything out there currently and to come.
Xbox Live costs money, Windows Live Free!
Xbox games $60. PC games $50. PC games also drop in price way before Xbox games. You will be able to buy your games for $20 while they are still for $40 on xbox.
You won't need to buy 3 additional controllers for your friends. You can buy a xbox controller for your PC and joysticks if you are a flying junkie.
Xbox can't play xbox360 games. They force you to go spend another $400+ on a completely new system in order to play the new games. PC DOES NOT. You may need to set the graphics from High to Medium. But you can play the game untill you have $180 to upgrade your graphics card, CPU or Ram.
The PC can play on your HDTV or computer monitor. You can use mouse and keyboard or xbox controller. Its tailored to how you like to play.
Basically what i'm saying is PC gaming is not has expensive as people may think it is. If you stay moderate in your purchases and get something that is great today and good for tomorrow you will probably save money going this route to your gaming.