Podcasting is so much more fun with guests and cohosts. These people can't always be onsite, so that's where a Skype or telephone call can bring them into the recording. There are plenty of expensive ways to do this, but I recommend a Mac Mini.
Mac Minis can be purchased for under $600 from Amazon.com or Mac Mall. I'll not be naive and say that $600 is cheap—it's not. But if you decide you want to add another computer to your setup, the Mac Mini makes a great, versatile option.
My particular Mac Mini is several years old and sat unused for quite a while. I earned it through one of those “free ____ if you sign your life away or spam your friends” kind of sites. (As you can see, they are sometimes legitimate. I also earned my first iPod, which I then sold, from a sister site.) But this Mac Mini was primarily an unused media center until I started podcasting.
Now, this Mac Mini handles my live call-ins via telephone (with Google Voice) or Skype. If it was new enough to run newer OS X version, I could do more with it.