why would you like to overclock it?
tell me what programs do you use?
it think a nice and big memoryupgrade could solve a lot
a friend of mine has a 2X2Ghz and added 3GB of ram in it, were both in the publishing industry, he as an graphic designer, I'm a photographer...
since I used his once, I knew with all that ram it has the power I will buy soon, because I make scans wich are 715mb, and my poor old PB has to think a long time on one (29/30 min/scan to make it and allso 10min to open one in Photoshop, nevermind thinking of doing a lot of filters on it), and he opened 5 of my files at once, was able to work on each one while they were all opened and it has the speed as if I use 5 files of 10mb...
want even more speed, change your bootdrive, put the 160gb as secondary and buy any 10.000rpm SATA drive (sizes vary from 36 until 80 GB I think) and this will add all up to the speed of your G5 because a lot of programs are more in need for extra ram (or like in the case of my scanner higher drivespeeds) than extra Ghz from the processor
or add an equal drive in the second bay and raid-stripe them, it makes the disksize lager and adds up to the writing speed of the drives since your computer writes on both at the same time...
and if your not satisfied after all this... wait for the 2x2,5Ghz, wich in my case won't be faster since my programs are more RAM and disk dependend in the end...
greetings & goodluck
J