Hi,
One thing to remember is that if you put the small collector right IN the window opening, you gain some heat from the small collector, but you lose the heat you were gaining from the window. South facing windows are actually good solar collectors, so you don't want to block them. A south facing window that has an insulating shutter or shade to reduce heat loss at night will perform just as well as a good commercial solar collector.
A small collector is a lot of fun to build, and I bet it will convince you that the idea works -- nothing like feeling the hot air coming out of the exit vent to make one a believer. But, it won't do a lot toward reducing your heat bill -- you need a good sized collector to do this.
If you have a lot of south facing wall, but are short on funds to spend on the collector, I just added a link to a really, really, really cheap collector that was published in Mother Earth News in 1977. Its here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
Look for: "Very Inexpensive Solar Shop Heater.."
The one in the article is very large, but you could build a smaller version.
With a bit of scrouging it can be built for about 10 cents per sqft!
The one I use on my shop only costs about $3 per sqft, and does not require any fans or controllers -- a small version of it might also be a good option:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
There is going to be an article on it in the Dec/Jan issue of MEN, along with an article on commercial solar air collectors by Dan Chiras.
Gary