The NEW G4s are hovering around $1400 dollars.. A steal for a powerfull Computer, no doubt. Sure, you could buy a 450 and upgrade it, but after all is said and done there, you're going to be running pretty close to a retail new G4. So unless you really have your heart set on hotrodding the PowerMac yourself, go for a New G4. Or Try to find one on ebay, they're going like hotcakes!!
Good luck!
Drew