Nice that you actually priced a build at home PC configuration. As you Can see the Price comes up to about the same For a new Power Mac G5 2.5GHZ!! at least here in the states it does, I also noticed that your pricing for the Opteron System is in USD so it's the same price at least here in the US.
The Power Mac G5 still has the clear advantage as the Build Yourself Dual Opteron System would still lack a Comprehensive Bumper to Bumper Warranty, Would still lack Toslink Digital Audio Connectivity, Would still lack the FW 800 Ports, and using the K8T800 MOBO only supports up to 2GB of Memory Max while the G5 can support 8GB!! Also with the 2.5Ghz G5 you get the Radeon 9600XT w/128MB of Ram. Not to mention the cool factor oh and I think the 500MHZ advantage the G5 2.5GHZ has over the Opteron 246 @ 2GHZ more than gives the G5 the Crown not to mention a 1.25GHZ bus over an 800MHZ bus!! Also as far as you saying the Opteron 246 would be faster compared to a 2GHZ G5?
http://www.barefeats.com/g5op.html
True benchmarking is subjective but I would say performance is a wash between the two! Note that now we have 2.5Ghz G5s and The fastest Opterons still top out at only 2.4GHZ I would love to see benchmarks in comparison to the newest Opterons and newest G5. I also think that the Mac OSX platform is overall better at MP Apps as the Mac has a greater amount of MP aware consumer Apps than is available for Windows or Linux.
So that's it I dare anyone to build a Dual Proccessor PC for Considerably less and EQUALLY equipped to the POwer Mac G5 in every way. IT is imposible I invite anyone to challenge this.