This diagram shows how I have the USB and serial connections layed out in the observatory. I was concerned at first by the idea of having a single USB cable coming into the computer but have found no issues. This is probably due to the fact that the main cable coming from the USB powered hub is an active extension cable and also that the hub is fully powered. So far this has worked well with no issues noted in dropped frames or download delays.
Thought I would throw in a little more info on the specific parts in this diagram. If you are thinking of putting together this same setup this should get you started. The links will probably go out of date pretty quickly but it should give you an idea at least on where to go.
7 Port Belkin USB 2.0 Hub - $49 at BestBuy
http://www.bestbuy.com/site/olspage.jsp?skuId=8488772&st=belkin&lp=3&type=product&cp=1&id=1186003682248
The USB to 4 Port Serial hub - $70 at CableMax
http://www.cablemax.com/serial-adapters/usb_4-port-to-serial_converter.html
Active USB Cable (this simply means it has a built in signal amplifier but must be for 2.0, 1.1 will not work with a 2.0 port) - $17 at Sewell Direct
http://sewelldirect.com/usb2extensioncable.asp