It's a good question!
Search Google for "real estate software" (no quotes) and there are 55,800,000 sites listed. Ergo, you have a real challenge in front of you.
Nevertheless, the answer lies in the fundamental principles that effective
SEO and SEM are based upon:
1. The right title, the right content, the right in and out links (i.e.
relevant ones), and -- all concerns about the Jagger update/upset aside -- a sufficient understanding of why the present Top Ten are where they are.
2. A focus on just what it is that makes your product different from or better/cheaper/simpler/faster than or more complex/advanced/sophisticated than or whatever else it is that applies. In other words, what is your Unique Selling Proposition -- and you'd better have one and
know what it is!
3. Identification of exactly who could/should buy your software. When? For what reason(s)? In preference/opposition/competition to what?
4. Identification of which directory-like sites serve the real estate industry with support materials such as software -- and this is a whole lot different group than the innumerable directories that specialize in listing agencies and agents.
5. Systematic and ongoing marketing to such sites (as in paid and unpaid submissions).
6. Perhaps most of all: patience. It ain't gonna happen quick (if only because the dreaded Sandbox is almost certainly going to get you).
And apart from all the above, you'd be well advised to take a decidedly serious look at the argument for using Adwords (and that rather than Overture). Done right, there's no quicker or less expensive way to get a new product off the ground.
Duncan