Hi, Danny, good to see you round here.
Kevin F said it all pretty much, the first two are especially render heavy. You might also consider using 'ray trace objects', found in the render node. This will disable the object quality issue completely, making your AA the overall object quality control. The population quality drop-down selection will then have no effect at all and needn't be worried about.
You'll see in the atmosphere and cloud nodes where you 'enable ray traced shadows', there is a description beside that check-box with the ideal settings to use that feature. Generally, if you need a sold object to cast shadows onto that particular layer. Here, I'd say you don't, it'll just increase your render time significantly, as will GI surface details. A lot!
When you say 'cloud quality=64 and 40', I hope that is the sample count and not the overall quality slider value you mean! I'm sure it is because, we've been over that one before, remember?
If you use 'ray trace atmosphere', like Henry suggested above, you could easily get away with lowering cloud quality to about '0.25', saving more sampling time still. Using atmo samples of 24 to 32, instead of 64, should be enough for this scene using the ray tracer with good AA settings.
After reading Oshyan's 'render settings recommendation' thread, read this one from Matt on using the ray-tracing options;
http://forums.planetside.co.uk/index.php?topic=8300.0
