Seeking Power Mac Recommendations?

Pauly asks: "I'm going to do something I've never done: I'm buying a Mac. I'm a long-time UNIX developer and user for whom the prospect of trading a noisy, heat-spewing workstation or two in favor of a civilized Mac is irresistible. I'm quite used to making x86/*NIX hardware decisions. However, I'm finding myself stumped as I look at the G4/OS X options."

"As attractive as the (i|e)Macs are, I'm pretty certain I'll be getting a G4 tower to re-use my subtantial dual-monitor investment. Specifically, for my needs as a developer and power-user, I wonder:

• Are modern G4 towers quieter and/or cooler than comparable x86 workstations?
• Is it wiser to spend money on memory or megahertz?
• Is it best to buy everything directly from Apple, or just a minimum to be fleshed out with cheaper, after-market add-ons?
• What's the best video option for dual-head on Jaguar?
• Does OS X make SMP worth the investment?
• Is the SCSI performance gain great enough to be worth the investment over IDE?"