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Who knows what lurks within the dark heart of the Frankenmac?

Running a Mac operating system on hardware not designed at One Infinite Loop may seem exotic to some Mac users, but it's a stroll down memory lane for me. Back in the 1990s, I spent about a year-and-a-half as an employee of Umax's short-lived SuperMac clone division. That was, of course, before Steve Jobs returned to Apple in 1997 and put an end to the Mac clones.

There's been a clone revival of sorts in recent weeks, sparked by Psystar's announcement that it would offer a low-cost PC capable of running Mac OS X. (Yes, Macworld Lab has placed an order for Psystar's Open Computer to see if the shipping product lives up to its claims.) Closer to home, Macworld senior editor Rob Griffiths built his own do-it-yourself Mac from off-the-shelf PC parts. And after running some informal tests, Rob sent his so-called "Frankenmac" to Macworld Lab for the definitive word on how its performance compares to an Apple-approved Mac.

Rob's original article covers the specifications of his home-built machine in greater detail, but here's a quick rundown of what we tested: Rob built a mini-tower computer with a Quad-Core 2.4GHz Intel processor, a 500GB Seagate SATA2 drive, 4GB of 800MHz DDR2 RAM, and an Nvidia GeForce 8800GT graphics. The total cost of this machine was $950 (not counting all the sweat equity used to assemble it)--a little more than what you'd pay for a 2GHz Core 2 Duo Mac mini after upping its memory to 2GB and opting for a larger 160GB hard drive instead of the standard configuration.

Looking at the performance numbers, however, Rob's cobbled-together Mac performs more like a higher-end iMac, and in some cases the scores are even closer to an 8-core 2.8GHz Mac Pro. The Frankenmac earned a Speedmark 5 score of 222, about 7 percent slower than the 2.4GHz Core 2 Duo iMac--the model released last August and not the revamped version Apple introduced Monday. The Frankenmac was about 9 percent faster than the 2GHz Core 2 Duo iMac, also released last year.

One area in which the Frankenmac failed to live up to expectations was in our series of hard-drive tests. Rob set up the machine so that three OSes were running on separate partitions on the machine's 500GB drive, which probably affected performance--a less crowded drive might have netted higher scores.

In more processor-based tasks, like Compressor and Cinema 4D, the 4 processing cores of the Frankenmac helped it beat both iMacs handily, approaching the performance of a Mac Pro.

In terms of gaming, the Frankenmac was able to display more frames per second than the iMacs and Mac mini in both Quake 4 and Unreal Tournament. It just about tied the 8-core 2.8GHz Mac Pro in our Quake 4 test, but it lagged behind the Mac Pro in our tests with the older Unreal Tournament game tests.

Do those numbers mean you should run out and assemble your own Mac? Rob's warnings about the amount of effort required, the fistful of warranties you'd have to negotiate, and the aesthetic compromises you'd have to make should scare off most Mac users. That said, this particular build-it-yourself system definitely delivers impressive performance, perhaps suggesting how a mythical mid-range Mac minitower would perform if Apple ever released one.

We now plan to turn our attention away from the Frankenmac toward machines actually built with the Apple seal of approval. Look for benchmarks of the new iMacs in the coming days. And as soon as our order from Psystar arrives, we'll test that system, too.

[James Galbraith is Macworld Lab director.]