He's already got the new machine (see previous posts in the thread about it being delivered). ;-)
I guess it's all a matter of perception and what you plan on doing to images.
If you look at some of the CS4 benchmarks at places like Tom's Hardware, the Core i7 920 is about twice as fast as a 3.0Ghz Intel Core 2 Duo (which is faster than the fastest CPU offered in the Mac Mini) at doing things like applying some filters to a tiff file. In the benchmarks used for Tom's test, the Core i7 920 completed them in 120 seconds, versus 241 seconds for a 3Ghz Core 2 Duo (a faster CPU than Apple offers in the Mac Mini):
If you look at some of the video editing benchmarks (which is something else the OP is interested in), then the Core i7 920 pulls away even more. For example, using CS4 Premiere to edit video, the Core i7 920 was roughly 4 times as fast as a 3Ghz Intel Core 2 Duo (which is again a faster CPU than Apple offers in that box), with the Core i7 920 completing the benchmark in 70 second, whereas the 3Ghz Core 2 Duo took 278 seconds.
In order to get a machine with roughly the same processor specs and ram in the Apple lineup, you'd need to move up to the Mac Pro and use a single 2.66Ghz Xeon (they use a Nehalem type Processor in it, with the same basic specs as the Intel Core i7 920), bump up the RAM to 6GB of DDR3 (although the Apple is using slower memory) for around $2700 (which a much slower GT 120 video card compared to the one the OP ended up with). If you want to stay Nvidia, you'd have to use multiple cards in the Mac Pro (since they don't appear to offer better Nvidia cards from what I can see from their configurator).
IOW, you'd be looking at around twice as much money for an Apple solution compared to what the OP paid for the box he bought (assuming he could even buy the Apple Mac Pro at U.S. prices), and he'd still have a slower machine given using the same operating system on both (since the Apple is using slower memory, with a weaker video card in it's base config trying to come close to what the box the OP has in it).