It's a safe bet that you won't find more food for less money than at Old Spaghetti Factory tonight. To celebrate its 40th anniversary, the downtown Minneapolis restaurant will be virtually giving away dinners for about $3--which includes a soup or salad and bread, a spaghetti entrée, and a scoop of spumoni ice cream for dessert. Kids' meals will go for about $2. If you miss the festivities on Tuesday, or if the lines are out the door (it's first come, first served), the restaurant will also be selling dinners at 40 percent off every Monday and Tuesday for the rest of January. The national chain is a long way from haute cuisine, but it serves perfectly acceptable grub, lots of it, and it's very family friendly. And did we mention IT'S $3 A MEAL?