Low tech/low cost foot warmers: 1 pair of "regular" cycling socks, one pair of wool cycling socks and one plastic bag large enough to fit your feet in (large lunch bags or those old newspaper bags work well.) Put the plastic bags in between the regular cycling sock with the wool sock as the outer layer. I rode these all winter last year and never had cold toes. Granted, it was a warmer than average winter, but if you'll only be riding down to 40 degrees, it'll do fine.
One thing you should definitely do is stop wearing t-shirts or anything cotton. That's probably been mentioned already, but cotton clothes get wet and stay wet. Not good. If you're actually going to be riding with any frequency, invest in some cycling specific gear or at least some running gear (tights and a base layer) to wear under cycling shorts and jersey. In 40 degrees, you probably don't really need your legs completely covered - you can get away with cheap knee warmers. And if you tend to overheat, the thickness or number of base layers would probably be lower. Personally, I usually go by feel -- I woudl rather start off cold because I know I'll heat up (and often times, so will the day if I start riding early.) So I usually try to go with a thin windproof jacket as my outermost layer and then take that off when I start to feel warm, but before I sweat.