benacheson - 8:03 pm on Sep 8, 2010 (gmt 0)

A sitemap can contain up to 50,000 URLs.

There is no need for a sitemaps index file unless you have more than 50,000 URLs.

[google.com...]

I seriously doubt you do have more than 50k. But if you have more than that they you could indeed use a sitemap index file.

I would recommend against having such a vast number of URLs from an SEO point of view, unless you are an authority site, as it stretches Pagerank. Make sure you're using rel="canonical" too.