Looking at what your options are for CS4?
Of course there is the Adobe Store and it’s very comprehensive and built around a Flex/Flash interface.
However, the people over at Toolfarm.com have come up with a comprehensive upgrade matrix as well. In addition to being a great dealer of all things plugins, they work hard at providing excellent service and sales to the most important products to the creative person.
So, if you want to look at what your upgrade options are for Adobe’s CS4, I hope you’ll take a look at Toolfarm’s Upgrade Matrix