Hi ukgeezer
Google state themselves that the server side scripting language used plays no part in the ranking of the site, so use ASP, PHP - whatever. A problem IS caused when the dynamic url generated contains more than 2 or 3 parameters (eg '?')
Quite a few carts will now generate 'static' HTML pages for you via the control panel.
I have had good results for the carts I have implemented for clients. I use a cart called Cactushop (
www.cactushop.com) -Version 6 has just been released and this is fully XHTML & CSS. The 'PRO' version they sell will also generate 'search engine friendly' urls if it is still of concern for you. In my opinion this is not as important as it used to be. Make sure you page content and titles are as good as they can be.