I can understand your situation, we got our car repossed due to a financial meltdown. To be very honest, I'd highly recommend going to a LBS (local bicycle store), usually if you buy from them they will do minor adjustments for FREE and since you'll be riding you bike in all conditions, that would save you a chunk of change over the months! Lite is good but it's more important to get a bicycle that "REALLY FITS" you and will do what you need. A commuter bike needs to be a bit "tough" and I'd recommend a wider tire as you can use it on all types of terrain/roads. I'm getting a $400.00+ dollar Jamis Trail-X 2.0 MTB as soon as my tax refund get's here (est. Feb. 17th.), this bike will be able to "do it all", including HAUL FOOD in my homemade bike trailer! For $800 you should be able to find a very capable MTB or Road Bike, might want to look at a "Cross-Bike", it's better on the pavement but still capable of riding a dirt path if needed. Specialized has a Cross-Bike, that has a 700cx45 width tire that should do you well, without going to a Fat Tire MTB or Hybrid.

Again, with the money you've got to spend, buying from a Bicycle Shop should be a good experiance and keep you up and running, FYI, YMMV PS. if you just have to go online, check out: http://www.performancebike.com