buying used...graphic design.

HD and RAM are pretty much stock. You can find all you need at macsales.com
Graphic cards need to be Apple specific, or rom-flashed pc versions.
A Mac Mini might be the best choice, though, depending on your needs. It's not really more expensive than a G5 and still compares quite well.

 

It does not only compare well. The current Mini's run circles around a 1.8 or 2GHz G5, by every means! (well except for hard drive IO of course )

A G5 with that clock speed is at least 6 years old and such an old machine simply doesn't work well with current software, especially when the software is optimised for Intel processors, which CS4 is.

If your main use case for that machine is CS4, I can guarantee you that you won't be satisfied with such a G5. If you desperately want a G5, get the Quad, it is the only machine that can keep up with the low end Intel machines of these days. If you can find a Quad for a reasonable price is questionable though.

So I definitely share the opinion of juanm, get yourself a Mini!

 

If you want CS5 then don't get a G5 based machine. The G5 PPC processor will not run any of the new Adobe CS5 apps as they are intel only.

Performance wise - the old G5 is pretty poor - my G5 Dual 2.5 is thrashed by later released Intel Macbooks, iMacs, just about the whole Intel range.