I use CS4 for everything. Looking to get CS5 real soon.
Hundreds of tutorials on the web for learing any version of Photoshop or any other software program out there. Youtube is a great start. Other forums specifically geared towards photography are also great resources. You can also purchase DVD's on forums, Amazon, or Ebay. Might also check your local Craigslist for people selling books or DVD's.
I only know about 1 percent of what is possible on photoshop. Mostly self-taught with the exception of watching some friends show me how they work an image. Haven't put the time into reading books, watching webcasts, or doing tutporials.
Don't feel like you need CS5 right away though I started with IPhoto which comes on all Mac computers. Purchased Photoshop Elements and hated it. Won the Olympus Pro Software in a photo contest, purchased Lightroom 2, and then went to CS4. For me it was a progression over 3 years to go from Iphoto to CS4.