5G? what are you talking about? I'd say that their $2499 ($3,899cdn) model is the best bang for the buck. That's dual. It would probably cost more than that to come up with a 2.53ghz P4 with a DVD burner from a major OEM offering a similar warranty. Their $1699 ($2,699Cdn) model isn't a bad deal either.
It's got:
Dual 1GHz PowerPC G4
256K L2 cache
& 1MB L3 cache per processor
256MB PC2700 DDR SDRAM
80GB Ultra ATA drive
SuperDrive (DVD-R/CD-RW)
ATI Radeon 9000 Pro
I was talking about the 1.25 GHz machines, but I admit, I would consider the dual 1 GHz as well. However, you should remember that 256 MB RAM is insufficient for OS X.2, and the thing does not come with a monitor. So add CAD$500 to the price at least. I wouldn't go with anything less than dual 1 GHz though, if only because of the psychological 1 GHz barrier and resale value. Also remember that this setup doesn't include MS Office either, just the useless Appleworks.
However, I can build a similar Windows machine with good quality parts for about >$1000 less, including Windows XP and MS Office. Mind you, I still like X.2 better and would love to be able to use iDVD. (I can't use iDVD on my laptop because iDVD is unsupported on a G3 and iDVD doesn't work with external DVD burners. Ironically, the pro program DVD Studio Pro works fine on an iBook.)
As for comparing dualies to single-chip P4s? Well, it was Apple who made the first comparison, but more importantly the reason the comparison is made is because they actually compete against each other. A dually G4 doesn't do as well in a lot of tasks as one might think. First, while the OS should do well, remember that even in OS X the apps should be SMP aware to best make use of the dually setup. Otherwise the software just run like they're on a supercharged single-chip system. Second, while a G4 is very fast on a MHz by MHz basis, a 1 GHz CPU just cannot compete with a 2.5 GHz P4. The P4 would blow it out of the water on most tasks. The 2nd CPU is required just to compete.
The PowerMacs are well thought out and well-made machines. However, I don't think that completely justifies the 35% higher sticker price. The reason people continue to buy them is because of the general OS experience and because of the software. Stuff like Final Cut Pro and DVD Studio Pro, etc. don't even exist on the PC side. The extra cost of the PowerMac is just an extra cost of doing business and that's fine for business, but for average Joe, a PowerMac is not necessarily a wise purchase.
Indeed, a single-chip iMac is much better for the average person. An 800 MHz iMac is CAD$3200, including DVD-RW/-R, 80 GB drive, 256 MB RAM, Geforce4 MX, AND 17" LCD. A similarly spec'd dual GHz PowerMac with Geforce4 MX and 17" display is CAD$5300. Even if you were a gamer, I'd still get the iMac, and spend the extra cash on a stripped down gaming PC to get something that would blow the dual GHz PowerMac out of the water in games.
IMO, today the PowerMac is for the most part a pro computer, not a general consumer product.