Your #container id has a 750px width and if you are going to combine the total witdht of #col1 & # col2 it exceeds to 750px. You might want to decrease the width of either #col1 or #col2 so that it equals to 750px. Also during your computation include the pixels that you have on your left or right margin and padding. So when when computing you shld follow this formula.
#container width == (#col1 width + #col2 witdth + #col1 col2 margin-left + #col1 col2 margin-right + #col1 col2 padding-left + #col1 col2 padding-right.
That should fix your misalligned sidebar.
– eidderf