I would never float two columns like that as it is just asking for trouble. Whenever you have to use more than 3 or 4 decimal points for a dimension you are asking a lot from browsers to get it right. Besides there's no such thing as half a pixel so browsers may round up or down to the nearest and you will be out by 1px - as in safari.
I would just float the first column and let the second column autofill the available space.
zoom:1.0;/* ie6/7 hack */
No gaps will be present as the remaining space space is always filled.