In our old house, we had the downstairs floors professionally refinished and decided to tackle the upstairs floors ourselves. Boy, was it a lot of work--but some great bonding (and fighting )time also. We had to strip the floors first. We used something that you would buy in the hardware store that bubbles the varish off. We had to then scrape it clean. Then had to sand the rest of the varish off. We rented a orbital sander and finshed sanding all of the floors in a weekend. We had to clean the dust up and off the floor. We did not stain the floor, but just put eyrothane (sp) on them. I think we did three coats. All in all, it took us three months of weekends to do about 800 sq. feet. I would more than likely pay someone to do the job again. Even when we moved out of that house (5 years later), we were still finding sanding dust from the floors!
It is definatly a do-able job, but it really it time consuming and a LOT of work.
Good luck!