"SEO URLS" have absolutely no influence on search engine spidering or your page ranking at all. nothing. zilch. zero. nix.
In fact, the search engines PREFER that dynamic url's are NOT re-written to static URL's, as doing this can result in the Search Engines finding "duplicate content", for which you will be penalised.
So rather than making your site more"SE-friendly", you could actually be harming your ability to be spidered and indexed.
MATT CUTTS - (Head of Google Search Quality Unit) has spoken extensively about this, and he does not recommend the conversion of dynamic URL's to static URL's .
Zencart is particularly good at generating URL's that Google likes, and when you couple this inherent capability with a good robots'txt file and XML sitemap, you will find that Google's bots rush around your site like happy busy-bees, gathering up all they need to know.
Also, with Google's new PANDA algorithm in place, your SEO focus MUST be on QUALITATIVE elements of your site, and your overall web presence.
Content quality, navigational link quality, external link quality and subject relevance are now THE most important elements of SE optimisation.
You should also work WITH the search engines, by having a membership accounts with Google, Yahoo and Bing.
So... in a nutshell, you do not need to put your site at risk by installing any SEO URL module. It will do nothing for SEO, except perhaps make it more of a risk.
If you want "reader-friendly URI's" for wordpress links, for example, then CEON's URI re-writer is a far safer bet.