get the bus speed that has the same frequency as your processor, that way when u'l want to over clock it... you'l get somewhere.
and yeah as geofelt mentioned - higher ram timings look impressive when under synthetic benching but show lil difference when in real world application.
if u game alot then they matter to some extent - but hey, get the 4gb with bus of less than 1600mhz.
since u'l put some stress on ur rig, wat brand u looking into?
i run windows7 64bit - 4gb will get u a long way