I went through a similar dilema. I chose the dual Power Mac G5. This response is being typed on it.
The reasoning is straightforward. The dual 2.3 Power Mac G5 that I bought is about 1.6x faster than a 3.0 GHz Pentium IV HT (based on every performance study I read). This is faster than the current Core Duo chips in the iMac/Mac Mini, and about the same speed (by my estimation, based on what I have read and Intel's public pronouncements on Core 2 Duo) as the new Intel Core 2 Duo chips will be.
So, if you are looking for power (forget the budget just now) eliminate the iMac and the Mac Mini. The Power Mac G5 is faster.
So, you want a Power Mac. Now, the next thing to consider is - shouldn't I wait until the Intel based Power Macs come out later this year? The answer to this is "no" for two reasons. First, they will be about the same "speed" (not in GHz, but in actual working results) as the current Power Mac G5s. So, waiting won't get you a faster machine. Second, that Intel Power Mac will have to use Rosetta interpretation for most of the major apps out there, such as Photoshop and Microsoft Office. As a result, they will be slower on the new Intel Power Mac than they will be on the existing G5 Power Macs. It will be about a year before the software vendors catch up with Apple and have universal binaries for all the major apps. A year is a long time to put up with slow apps.
So, you want a Power Mac G5.
...or you want to sit out the current year and buy the second generation of Intel Power Mac sometime in 2007 when it comes out (built using the successor to the yet to be introduced Core 2 Duo that will be at the heart of the first generation Intel Power Mac). This second generation Intel Power Mac will actually be faster than the G5s, and Universal Binaries should be available for most things by then.
I didn't want to wait. I wanted the power now, and thus am now happily driving this speedy new 2.3 GHz dual core Power Mac G5.
One last thought may be crossing your mind, which likely goes something like this... "but wait a minute, the G5 is dead ended hardware! Why would I buy dead ended hardware?". This couldn't be farther from the truth. What is the expected service life of the average home/office computer? Less than five years. I *guarantee* that over the next 5 year period, all new software wil come out as Universal Binaries, which means support for both Power PC chip and Intel chip architectures. For validation of this assertion, look to history - Apple's last core transition, from Motorola 68K to PPC. For the expected lifetime of the new computer you are now contemplating buying, you can expect to have access to every existing title that is out there now (without Rosetta interpretation) *and* access to any new titles you may wish to buy.
So, if budget isn't an issue, buy the G5. If it is, let your financial constraints guide you - technical has to take a second seat!