Exercise equipment?????

I usually jog 4 miles up to 5 x a week. I got pneamonia(sp?) about 3 weeks ago and are still on the mend. I have been thinking about buying a stationary bike for some time now but would like to spend a reasonable amount of money, and I am not quite sure what the benefits are of a bike. What area do they target? I thought maybe I could walk/jog 3 x a week and use bike 2-3 x a week. Any suggestions?? I excercise for health but also to keep weight off and to tone. Thanks in advance for any input.....