Product list dialog?
 

I was able to modify successfully product details page, bu I am having hard time figuring out what files to modify when it comes to the list of the products when i click a category or when I display lis of products by company. Could someope please point me in the right irection.

products.tpl for a single column display, products_t.tpl for a multi-column display.

Great. Now, when I modified my product page template I've created a copy of the page and renamed it. Shall i rename all pages I modify? Something tells me it would be a good idea...

Why do that? Just make a backup of the old one...much easier.

I was thinking if I ran an upgrade my tweaks might get lost. But I am glad to hear your suggestion, because that's exactly what I've been doing so far.

If you rename the file, you will never get it to upgrade correctly :)

True, but will the upgrade messup my code?

No, the upgrade will only upgrade lines that match the .diff file exactly. You will get a 'can not patch' error for any files that don't match, which you will then need to modify manually.