So you don't want to pay for a high powered computer, or you just can't afford to. Well there is no reason to worry or to sell of your younger/older brother (or the kids). All you really need to do it wait for OnLive to get going and you are good to go.
OnLive is a cloud based system that allows you to play games even on the lowest end hardware. To do this they execute all of the commands and code in a large power cloud system and send you only the parts that need to be displayed on your system. It is like a high performance version of the remote desktop protocol.
OnLive is currently in an invite only Beta they are still accepting requests for invitation so you could hear over and see if you can get in.
I will be very interested to see how this works out and what hardware these games will really run on. After all a service like this is not in the interest of the gaming or hardware industry at all.