Hi there,
I'm not sure about OsCommerce I haven't used it in a very long time. My personal thoughts are that it is not very search engine friendly without much time spent hacking it.
The example site you gave had url's like this:
http:// www. discs.ie/index.php?cPath=19&osCsid=da2e52cffc87eebd9e8865bc240ac8a4
Unfortuantely it includes what's known as a session ID see bold part. These are not
seo friendly because sessions expire or change for each user. This means that indexing them can result in a page not found later. Also sessions ID's tend to cause duplicate content issues.
A good shopping cart system would be one that outputs clean url's
http:// www.discs.ie /accessories/4-color-CD-DVD-Pen-Set.html
Instead of:
http:// www.discs.ie /product_info.php?cPath=19&products_id=26
Also the system should support XHTML output not outdated HTML.
If you want to appear in search results its important that each product has a unique title, description tag and page heading. The product should also include a good on page description.
You could alternatively look into:
Joomla!
Which has several shopping cart solutions, it can generally be made more search engine friendly. In my experience though I don't like either system much oscommerce and Joomla.
What payment solutions do you want? If you just need PayPal I could suggest a better CMS.