Winter Gardening

Well I have been getting antsy and I decided to start planting in the garden the other day.  It has been unseasonably warm for several weeks now and all of the extended forecasts I am reading are suggesting that we will not have a freeze for at least the next 30 days.  Risky, but I'm willing to take the risk at this point given the crazy winters we have had the last couple of years.  I ended up working over a few beds with a bit of compost, crushed egg shell and rotting leaves.  Thus far I have planted two varieties of radish (a white columnar variety and a red and white round radish) as well as spinach and kale.  All cold hardy things, unless we get prolonged freezes.  The bed they are in is fairly well protect from the elements so hopefully all will be good!  I'm looking into getting a greenhouse this spring so I can start many of the plants I want from seed with better success than last year.  Anyone have any good suggestions on economical units?