I want to take some photos from my house and give to my Realtor. I have a "Canon PowerShot SX20 IS with 5-100.0mm" which is pretty good for most of casual photography. I was wondering if that lens is wide enough for this job. I appreciate any suggestion, I can spend money on an SLR with a 18mm lens If I have to, but wanted to know if this works out.
In general, what you care about for real estate photography is angle of view, not focal length per se. The way things work out, the 5mm lens on your SX20 will have an angle of view which is very similar to an 18mm lens on an (entry level) SLR. The easiest way to compare these is to look at "35mm equivalent focal length", which is expressing the focal length you'd need on an old-style "35mm film" to get the same angle of view. For your SX20, this is easy to read from the spec sheet - 28mm at the wide end. For the SLR, the answer is that you multiply the focal length on the lens by 1.5 (or 1.6 for a Canon SLR) to get the equivalent focal length - so the 18mm lens becomes 27mm in "equivalent" terms - or in other words, pretty much indistinguishable in terms of angle of view.
That all said, the SLR will probably give you higher quality images than your SX20, so would still have some potential advantages. On the other hand, you could buy a relatively cheap point-and-shoot camera which would have a wider angle lens than your SX20 (just as an example, the Canon SX50 has a 24mm lens, which is significantly wider), or you could just pay a professional to get some photos done if your realtor can't do it themselves; certainly in the UK you'd expect them to, but things may be different in your neck of the woods.
It's probably also worth reading What features should I be looking for a budget (under $300) camera for real estate? if you are thinking of spending any money.