Mac G4 versus G5 versus Windows XP

[progress88]

I really need some help with this decision. I recently purchased a digi 002 system with the intention of using a Mac G4 running OSX. Now I'm hearing that the new G5 is the way to go. I'm going to use my system for my own demos, recording my group, and songwriting now. But I'll probably start doing work for other people sometime in the future. I don't anticipate running 20 zillion plug-ins, but after I get into it who knows? I have a friend who runs Pro-Tools with Windows XP and swears by it. The four systems I was considering were:

1> Mac G4 DUAL 1.25GHZ 768MB ram, combodrive, 80GB HD ...............$1700.00
2> Mac G5 Single 1.6GHZ 256MB ram, superdrive, 80GB HD...............$1994.00
3> Mac G5 Dual 2.0 GHZ 512MB ram, superdrive, 120GB HD...............$3000.00
4> PC unspecified running XP 1024 ram,cdrw, 120GB HD.................$1000.00 approx.

I know that the Dual G5 will run faster, but is it worth the money for my applications? Money is definitely an issue hear.

My question is: If YOU were starting from scratch, which system would you go with in this application? Feel free to suggest changes to the above systems.

ps. I know I still need a external Firewire drive

Thanks in advance for the help,

Mike