Categories - set up for search engines or customers?

So I'm just setting up categories on our ecommerce site...

It all suddenly got a little confusing - I'm trying to strike the right balance between getting it usable for customers and also suitable for search engines.

We currently have around 400 products which could double in the next year or 2 and hopefully will keep on increasing

The two main issues are as follows:

- Category depth: From an SEO point of view I assume the main page should have a set of categories, and each of those cats should have sub-categories. Maybe a third level of categories??
From a customer point of view, it's nice for them to be able to drill down a few levels I guess to get to precisely the item they need.

These are my opinions only, based on guesswork, so can anyone suggest the optimum number of categories to suit SEO and customers?


- Multiple categories: This one is a little more annoying. Our ecommerce software will allow a product in multiple categories if we want. However, from a search engine point of view, they will be different pages with very similar content (only the H1, URL and page title will vary).

So what do I do here? My current thinking is to use the multiple cats. I'm hoping that search engines should pick up fine on one of the pages (and not penalise it in any way???) so visitors from search engines are catered for and once customers are on the site they will have the opportunity to see the product in a couple of different cats.


Anyone got any thoughts or comments on any of those?

Cheers,
Dave

I would focus more on making it usable for your customers. What setup makes it easy for customers to find what they are looking for and check out.. after all what good will your website be if customers don't buy anything. You can have a 1000 hits per day and still sell nothing.
As far as SEO in your catagories... just make sure the content is different. Then you need to work on baclinks to the various catagories to boost the rankings. You also need to really work on SEO for your main pages and make sure everything is interlinked.....

As far as a certain number of catagories... I don't think that really matters as long as everything is designed correctly. There are stores that have 100's of catagories that tank #1 and some that only have a few that rank #1.

use any text-based DHTML multi level menu as far as possible,

example
http://www.filamentgroup.com/lab/jquery_ipod_style_and_flyout_menus/

Absolutely focus on what you think will work for your customers. Then worry about SEO.

Some other people may be better able to answer your question about listing the same product in two places. However I do not think this will have a strong impact unless you are replicating most of your catalog several times over.
Google and the other search engines are well aware that sites need to list some products under more than one category so I think they tolerate a certain amount of this kind of 'duplicate content'.