First and most important thing is a Good Solid PC with a soundcard that provides you with ASIO low latency drivers.
Anything in the 2 Ghz range should do with at least 512mb of ram. But your soundcard will be the driving force. Have to ask yourself questions like....What will I be recording how many inputs do I need how many outputs do I need. A good card is Delta 1010
I use Cubase SX 2 but I also have Reason 3.0 which is an awsome app in itself but piece with Cubase or Sonar is all you will ever need for sounds! with all the free Refill out there.
Next you you need a good solid Mic.
All together you are looking at around $2000 but believe me with this you can Mix and Master Demos good enough for the radio.
Here is an excellent place to get you started with home recordings! GoodLuck!
http://www.tweakheadz.com/