I like the way your current cart displays sub-categories. I'm sure that does well on the search engines.
INVENTORY / INVOICING SOFTWARE
Which inventory and invoicing software are you using? Also, are you doing drop shipping on some of your products? What cart are you currently using?
While I am not familiar with the specifics of your operation, I tend to shy away from those commercial operations, since they then have a certain amount of control over you when you sign the dotted line and subject yourself to their fees.
Understand, my bias in favor of open source, Joomla and Virtuemart. I am currently working toward integrating a means of updating inventory and pricing via CSV from various accounting/inventory software applications. This is possible through a manual import, using CSV and/or SQL. Once I accomplish this, which isn't too hard to perform this operation daily or even weekly, I will look toward having this operation done automatically, but that is beyond my capabilities and specific to certain software applications and network parameters, which I do not know much about.