I was in a similar predicament last year -- new house, poor lawn, little clue about tending to lawns. There are probably more qualified readers who can give advice, but here's what I've learned.

It's probably too late in the season to reseed now. It's getting colder and the days shorter. Plus, with fall right around the corner, you're new lawn will be quickly smothered by leaves. Accordingly, I'd wait until the spring before doing anything to your lawn.

Now, you have two options: renovate the lawn or try to tilt the weed/grass balance in your favor through good cultural practices.

1) Renovation. This is a good option if you feel that you've got more weeds than grass. The steps are pretty easy and it's what I chose for my backyard, which was in shambles when I closed on the house. In short, renovation involves spraying a non-selective (read: it kills everything) herbicide like Round-Up. After a week or two, all plant life is dead -- grass, weeds, everything -- dead. At this point, consider amending your soil and ripping up the old dead lawn with a dethatcher, tiller, heavy-duty dethatching rake (if you're lawn is small enough), etc. Once this is done, your lawn is ready for seeding.

If you do chose the renovation route, the next question is one of timing. When do you do it? Spring or Fall? You're correct that fall is the best time to seed. I can attest to this. Unlike spring, new grass in the fall doesn't have to compete with the hundreds of thousands of weed seeds lying dormant in your lawn waiting for warmer weather to germinate. I tried repairing some patches of lawn last spring and was tough going. The great conditions I created for my new grass was equally great for crabgrass and other weeds. The result: I was more or less right back where I stared. That's not to say that seeding in the spring won't work, it'll just be a little more difficult. On the other hand, if you don't seed in the spring, you'll have to deal with you're crabgrass until next fall. One option would be to renovate and apply a pre-emergent crab grass control that's designed not to kill grass seedlings.

2. Good Cultural Practices. I used this in my front yard where the weed/grass ratio was in my favor. My goal was to make the lawn as inhospitable to weeds as possible. First, I applied a pre-emergent crabgrass control to prevent the germination of crabgrass. (As you may know, crabgrass is an annual. It produces seeds, dies completely in the winter, and germinates again in the spring.) This worked pretty well. It didn't prevent 100% of the crabgrass, but it easily got 80-90% of it.

Next, I raised the cutting height on my mower to its highest setting. The best defense is a good offense. A tallish stand of grass will shade and crowd out other weeds before they have time to get a foothold in your lawn.

I also fertilize with a weed and feed product that binds to the leaves of weeds. The results have been favorable in the front. I had a decent lawn to start with and have pushed out nearly all of the weeds in the front.

Conclusion. You have some choices. My advice is to fertilize this fall with a weed and feed winterizing mix. At the least, it'll provide a boost to what grass you do have. Then wait until spring and try to tilt the ratio in your favor and see if you can get ahead of the weeds in the spring through good cultural practices. If that doesn't work, begin lawn renovations in late August, about a week before Labor Day 2008. Lastly, consider picking up Scotts Lawn Care Guide. It's the antithesis of organic as it pushes the "Better Living through Chemistry" thing a bit hard, but it covers all of the basics. Also, Cornell's Extension Service has a great website on lawn care.

Good luck!

Here is a link that might be useful: Scotts Lawn Care Guide

I appreciate all the help. My backyard is actually much better than my front yard. The front yard is warmer, plenty of sun. The backyard is much cooler, lot's of oak trees to shade the area, so conditions are pretty unfavorable to crabgrass.
However, I've also heard that the best defense is a good offense. (football fan). If I seed well enough in the spring, it will crowd out any weeds or crabgrass. Isn't that true?

Seeding in the spring might crowd out weeds, but it will be a bit of a struggle. Here's why: Your new grass seed will be germinating at about the same time weed seeds will be germinating. As you water and fertilize your new grass seeds any weed seeds present in the soil will get the benefit of your TLC.

I've seen new lawns put in around my neighborhood in the Spring, so I'm sure it can work, I just had much better success when I did it in the fall. But, if you apply a pre-emergent crabgrass control that's compatible with new grass seeds (most crabgrass control will also prevent grass seeds from germinating), you'll probably be in business. I guess it's worth trying in the spring given that the alternative is waiting almost a full year to reseed!

Something else to keep in mind with spring seeding is that the grass doesn't have as long to get established before the heat of the summer, so more grass will die in the summer with a spring seeding than a fall seeding.

Dormant seeding may have better results than spring seeding, but you still have the issue that you can only use certain preemergents that will allow the desirable grasses to grow but suppress the crabgrass.