Hi Junior Salters, for comps, don't trust a Zillow estimate's. They are based on some fancy math calculation that apparently applies to the nation.
If you don't want to use a realtor, I would pop the addy into Zillow, click on the house so it brings it up and on the bottom right side (scroll down a good ways), there is a "Nearby Similar Sales". Click on that and then try and find similar spec'd houses. This is probably going to be your best tool without using a realtor. I just re-read your post and you may already be doing this. There are advantages to having a realtor since they watch the market a lot more, can send you the comps in a nice list with pictures and all of that. Your going to want to look at sold houses from the last 3 months. You can also use Redfin to see whats pending. You might be able to do this in Zillow to, but I find the Redfin mapping tool easier to use. Also, I don't know if Redfin is in your area yet, so if not, you'll just have to figure it out in Zillow. I wouldn't use pending as real comps, but just a way to see if things are selling in the area. I know Redfin shows you how many days on the market as well, so that is helpful for a comparable, but a pending sale doesn't tell you exactly what the house is selling for, but if it went into 'pending sale' in a few days, its probably going to be around the list price.
Realtors have better tools with more filters, but this is pretty nice if all you want is just a quick comp. Also, most realtors won't respond back fast enough when you send them 10 properties to comp out when all you want is a quick analysis.
If you are providing a realtor with some sort of way to make commission, then there is no reason why you shouldn't use a realtor, especially if they can send you leads that they come across (bank owned, stuff on the mls). If you are providing a realtor with nothing, meaning your going to get the comp and then wholesale it off and be done with it, then you will find that they will slowly never respond back to you.
Hope that helps!
Joe