Umm.... maybe you had better get a reality check.
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2388&p=12
The Pentium Extreme 840 running with Dual Cores @ 3.2Ghz encodes a DVD in 7.4 Minutes. My Dual 2.5Ghz G5 can encode a DVD in about 8-12 minutes. Still faster than a single Athlon 64FX-55 and just a couple minutes slower than 700Mhz faster Dual Cored 3.2Ghz Pentium Extreme 840. I would guess that a Dual Core or Dual Processor G5 @ 2.7Ghz would come even closer in score to the Extreme Edition 840 and most Certainly match or beat a 2.8Ghz Dual Core Pentum D.
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2388&p=9
I can also convert a 12Mb .wav file to MP3 192KBs with itunes 4.7 in under 5 seconds while the Dual Core Pentium Exreme 840 takes 29 seconds? Maybe this is an unfair test though since it's Apple software on an Apple Machine. However My DVD compress was done with DVD2OneX which is not Highly optimized for SMP.
http://barefeats.com/macvpc.html
Here is a link of how the Dual 2.5Ghz G5 compares to the Dual Processor Opteron 252 clocked at 2.6Ghz, Dual Xeon @ 3.4Ghz and an Athlon FX55.
As you can see the Dual Processor G5 matches or beats the PCs in almost every scenario. The Pentium Extreme 840 and Pentium D are not much faster if at all in most scenarios than a Dual Opteron or Xeon system. Also keep in mind that Tiger is supposed to offer a nice speed bump in most areas and these tests were done under Panther 10.3
I do admit though that the Mac performs horribly at Games in comparison to the newest PCs but hopefully that Gap will become smaller with Tiger and future driver updates. In most things besides Games the G5 PowerMacs can compare reasonably well with top of the line PCs.
I also Don't think you quite understand that having two single cored Processors Performance wise is almost the same as a Single Processor with two cores assuming similar clock speeds and Core. So a Power Mac with two seperate chips running at 2.7Ghz should perform on par or beat a Single Pentium chip with two cores running at 2.8Ghz. Remember the G5 is still faster clock for clock than even the newest Pentium Cores. The Pentiums Two cores share an 800Mhz Bus as well while the 2.7Ghz G5 machine is supposed to have two independent busses running at 1.35Ghz. If you were talking about Dual Processor Dual Cored PC's you would have some point of validity with Quad core. However the Intel Extreme and D as well as the upcoming Dual Core Athon only support one Dual Core Chip. Sure the Dual Core AMD Athlon is still socket 939 however it still supports only one Dual Cored Chip until the Dual Cored Opteron 265/275 series is available at a whopping $850-$1300 per Dual Cored Chip. So one can have a quad cored Opteron for about $2000-$3000 just for the CPUs and MOBO as most A64 and FX support only one 939 socket.
At the rate we are going we most certainly will see a Quad Cored 970 MP setup available from Apple hopefully by the end of the year. Apple's CHUD tools allready support controls for a Dual Processor Dual Cored system. I think you underestimate how far the Mac has come within the past couple of years.