For starters, I'd suggest you go with clip pedals rather than straps. That way you can wear more on your feet without worrying about getting it snarled up as you try to stick the toes in. You also reduce the metal-on-shoe contact, and metal does a lot to conduct cold into your feet.
With or without clipless pedals, here's what I'd do: buy a new pair of shoes that are a half-size bigger than what you wear now. That will give you more room for socks, such as SmartWool's Light Hikers ($14, www.smartwool.com), a very warm and comfortable sock. Over those, put on a pair of Gore Windstopper Socks ($36, www.gorebikewear.com), which are really wonderful, warm, windproof socks. Your current Gore-Tex socks will work well, too, although your feet will breathe better in the Windstopper socks and may get less sweaty, something that can chill them as you cool down.
Then, your shoes go on. Make sure your feet fit comfortably, as if the shoes are too tight they'll make your feet colder due to loss of circulation.
Finally, you'll want some kind of overboot. I'd suggest Pearl Izumi's Toester Bootie ($60, www.pearlizumi.com), made of fleece-lined neoprene and extremely warm to boot, pardon the pun.
If your feet get cold with all that, then it's time to go home.
Read the "Essential Mountain Biker" from Outside's 2004 Buyer's Guide for a rundown of the mountain-biking gear that's, well, essential.