Google found these for upgrading the CPU:
[http://www.ambor.com/public/meromswap/meromswap.html|http://www.ambor.com/publi c/meromswap/meromswap.html]
[http://macminiupgrade.com/upgrade.html|http://macminiupgrade.com/upgrade.html]
Apparently the 1.66GHz models have a CPU in a socket, but later models the CPU was soldered. However, the articles do NOT mention whether you will still get 64-bit support in an upgraded Mini. As Boece is inferring, the Mini's firmware or other components may not allow for 64-bit functionality since Apple never sold your model Mini that way. So while it appears to be technically possible to upgrade the CPU in your case, you still might not be able to run CS5.
By "CS", I assume you mean Adobe Creative Suite. If so, note that not all of the components of Adobe Creative Suite require 64-bit. For example, according to [Adobe's webpage, Photoshop CS5 will still run on 32-bit|http://www.adobe.com/products/photoshop/photoshop/faq>. So if you only need just one component, then you could buy just that program instead of the whole suite.
However, considering that you'd still be limited in RAM, IMHO you really should be considering replacing with a newer Mini (or iMac or Mac Pro.) I find that people who "need" Photoshop and the rest of the CS product line, tend to also need more RAM. It is quite possible that your Mini's firmware will limit you to 3GB of RAM. So selling your Mini and buying a newer model is probably a better investment. (And when does Photoshop and it's brethren
not need more CPU speed as well as RAM. 😉 )
(edit: sorry, I missed that you mentioned Adobe Premiere, which does require 64-bit.)