The primary difference is that it'll run better on Intel based Macs. That was the primary reason for CS3, I believe. CS2 (along with Macromedia Flash and Dreamweaver) wouldn't run as smoothly and their performance wasn't as great with the Intel based Macs.
Also, Adobe bought out Macromedia, so Flash and Dreamweaver are apart of Adobe now (another reason CS3 came out). So, old Adobe products like GoLive are no longer there. So, from what I've read, there's a better integration between the programs, such as Dreamweaver and Photoshop that there wasn't before.
There's more here
to read up. There isn't a HUGE change between versions, it was mostly due to the fact that Macromedia was bought out by Adobe and that Macs had Intel processors and CS2 often crashed on them (It pissed off some of my professors in college last semester when they got the new computers).
If you have a Mac, you might want to either upgrade or go to CS3. If you have Windows, you can stay with 8 unless you'd like to go to the new version.
I hope this helped!
"I would feel more optimistic about a bright future for man if he spent less time proving
that he can outwit Nature and more time tasting her sweetness and respecting her seniority."
E. B. White (1899 - 1985)