The lettuces, spinach, kale and cabbage would be fine out in a coldframe. I've been growing all 4 of those all winter long. The spinach has been doing quite nice. The warm season stuff cucumbers, tomato, squash and melons are okay in the coldframe but not until 2-4 weeks before your last frost date. If you put them out earlier than that they may get too cold at night and become stunted. Unless maybe you want to have a heated coldframe which is them called a hotbed. I use Christmas lights for extra heat. I thought about the buried cable option but I have a way of accidentally digging up things that are buried. As digdirt said coldframes/hotbeds are something that one needs to monitor every day. I use 4 different methods. The frames wide open for days above 40F, cracked open about 3 inched for days 30 to 40, cracked open 1/2 inch for days 25 to 30F and closed fully below 25F. Just harvested a bunch from this frame. :-)

After checking around a bit, I guess prices do vary. But both the Clean-Ace and Clean-Arc can be found for $70 or less if you know what to look for. The Clean-Ace is actually a conversion bulb meant to run on a mercury vapor ballast, but many people (myself included) run them on MH ballasts without any problem. The Clean-Arc is meant to be run on a MH ballast, and is the bulb I would have chosen if I hadn't found such a good deal on the Clean-Ace. I ordered (but haven't yet received) the Clean-Ace from Salty Supply. It's called an Iwasaki 6500K in the aquarium industry. (EYE is a subsidiary of Iwasaki.) Doing a search for 400w Iwasaki will turn up lots of $70 bulbs that are in fact Clean-Ace bulbs if you check the model#. The Clean-Arc can be found for $70 at Business Lights as well as other places. As for the specs, you're looking at the base up/down (BUD) version of the Clean-Ace. The horizontal (HOR) version has identical specs to the Hortilux Blue, which is also a horizontal bulb. T15/17 may refer to the shape or size of the glass portion of the bulb (not sure), but they all fit in a standard mogul socket. IMO, this is just another example of charging different prices to different markets for the same product. I've been very happy with the 250w Clean-Ace in the past, and I'm sure the 400w Clean-Ace/Arc are just as good.

I start my seedlings indoors under a shop light in the peat pellets and move them to a cold frame, when I trensplant them to pots. I also keep a fan circulating the air indoors so the plants get used to it and grow more sturdy. I used christmas lights as heat source in my cold frame, when the night temperature got too close to freezing.

Tammy - first things first, lettuces, yes, you can grow them in a coldframe now. I have done this for years, no problem. As for tomatoes, I am trying them this year in a coldframe. I plan to start them indoors around feb 1, and then transplant them into the coldframe around Mar 15, much earlier than my recommended safe day of May 1 (note, I am just east of you in zone 7A (N. VA) so you may want to adjust your schedule accordingly. As for how the coldframe works, in daytime, its essentially a mini-greenhouse. As the sun streams in, I can achieve anywhere between 20-25 degrees over the outside temperature. This is fine now, but come late mar, early april as we sometimes reach 70+ degrees, the issue then is too much heat. In years past, I literally cooked my tender greens by neglecting to uncover them on warm days. Thus, as potterhead notes, its not a bad idea to get an automatic vent opener (it operates as temps cause a cylendar to expand and lift the lid). My bigger concern now is nightime temps as I dont want it to get too much below 50 degrees at night and potentially stunt production. In lieu of christmas lights, I am using an adjustable spotlight, facing the ground with a 150 watt bulb on a timer. I dont know if this will work, but thusfar, I am able to achieve about 10 degrees over the nighttime temps, possibly more. That said, the 150 watt light seems awfully hot - im almost afraid to use it for fear it will set some mulch on fire! Any event, these are my plans. But again as far as lettuce goes, go for it - just be sure to somehow vent it if necessary.