Regarding the column that's dropped down, it's a common IE problem. Because the two div's must touch, IE forces one onto a new "line" as it were. The only way round that I've found is false columns.
Keep #rightcolumn @ 171px wide floated left but make #maincontent 660px floated right. The put the background on the contentwrapper div instead of on each of the two.
Hope that made sense, it probably didn't.
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