Are you looking to increase search engine spidering of your site?
The first thing I would do is edit meta.tpl which includes a bunch of x-cart's search terms. Next thing I would do is not worry about meta tags. Search engines learned quite some years ago not to trust webmasters and so they evaluate your site themselves and use all kinds of advanced heuristic analysis of your page content vs. other pages' content vs. position / prominence / special tags / image alts / etc. It's a huge statistical nightmare.
For example, I went to google and I typed in "dirt bike". The second page that came up was this piddly AOL page with no meta tags and no real traffic at all. Why is it number 2? I can't tell you, but I do know that meta tags had nothing to do with it.
Try increasing your keyword prominence (put them in tags like [b], [i], <h3>, etc.) and adding keywords to your image alts. Then make crosslinks in your site like crazy. And having a site map doesn't hurt either. You want to give the search engine a big juicy pile full of links to chow down on while you're off making money.
Also, with google, you'll want to get as many incoming links as possible, especially from related sites. A lot of good info can be found on sites like
www.search-engine-secrets.net and
www.searchenginewatch.com. I also like Web Position Gold (
www.webpositiongold.com) for reporting my site's ranking in the top search engines.