Here's a comparison using info from both the AMD website and the Apple website.

Test Systems:

Apple G5 dual 2GHz and AMD Opteron 244 dual(1.8GHz)

SPECfp_rate_base2000 (G5)15.7 (AMD)24.7 (% faster)57%
SPEComt_rate_base2000 (G5)17.2 (AMD)25.0 (% faster)45%
SPECfp_base2000 (G5)840 (AMD)1122 (% faster)34%
SPECint_base2000 (G5)800 (AMD)1095 (% faster)37%

One thing in the defense of Apple... the dual Opteron boxes are going to be very expensive, and are targeted at the server market (no AGP bus). Although I hear that the nforce mobo coming for the Opteron will have 8x AGP. I would say that price wise they're very competitive for a change, and for $3000 you'd be hard pressed to find a PC that can compete with their published performance. I bought a high end Dual Xeon Dell box not long ago that would beat up the Apple... but it also cost almost 10g's.

I just priced a 3ghz Dell system with all the bells and whistles I want (Dual channel ddr, Radeon 9800 Pro with second monitor connector, 120gb SATA, Audigy, 17" flat panel ... $2800. The G5 looks pretty darn competitive.

It's the first Apple I've seen that I'd consider buying, but sadly I'm too M$ commited to do so... my next PC will probably be bought to kick Doom III butt, and all my buddies are PC centric.