Upgrade from OS9 to OSX

Hi Guys

First of all im a total Mac newb, so im going into this a bit blind

We have a client who has 2 G4s running OS 9.2. They are a Graphic Design company and use mainly Adobe Illustrator, Photoshop, and Quark. They want the machines upgraded to OSX. Ive done a bit of research and apples website describes the process as being pretty straightforward. I know better than to beleive stuff like this!
Id like to know if there are any issues I should be aware of regarding the upgrade procedure? Also, would it be best to do an upgrade, or an Archive and Install, bearing in mind the clients need all there data etc.

Thanks

Full back up and then an archive and install IMO, aslong as they have new versions of the progs that they are using then there shouldnt be any real problems

Keep in mind that OS X is very memory hungry, the more it has, the better it runs. 512MB is the minimum to be comfortabel (without too much swapping) but for professionals that work with Adobe soft and Quark, I wouldn't use less than 1GB.