Yeah, you definitely don't need a Quad to 3-box. I 4-box smoothly on a late '05 dual core 2.3 GHz with 2.5 GB of RAM and a GeForce 6600. I'm running Tiger (a bit more streamlined than Leopard) and play with old models, no spell effects, and so on.
Just to clarify, the late '05 line wasn't this:
PPC Dual Core 2.3 / 2.7 or Quad
but rather dual core 2.0 / 2.3 or Quad. The only dual 2.7 GHz was an early '05 speedbump with dual processors.
This makes shopping for G5 towers a little more difficult, since the '04 line has dual processor
2.0 and 2.3 GHz models, while the late '05 line has dual core
2.0 and 2.3 GHz models. The late '05 models have the other speedbump upgrades that Dimitre mentioned, so if you get some information from the seller about bus speed, max RAM upgradability, etc then you can pretty easily figure out which vintage tower you're looking at.
Really though it shouldn't matter too much. Dimitre's definitely right about the faceripping and throat$hitting.
P.S. the wiki page
on G5 towers is a good starting point for spec comparisons. If you want to dig deeper I'd check out the Apple pages.
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