MHZ Myth Sidebar:
I know people are not going to want to hear this, and I know I'll get flamed for it, but for a couple of years I was obssessed with this megzhertz myth and kept a running spreadsheet of dozens of processes run on both Macs and PCs that we used at work.
My conclusion was that, in general, the MHZ is far and away the number one factor in speed. And yes, an 800 MHZ PC was, in general, twice as fast as a 400 MHZ Mac. And, more or less, a G3 was to P3 as a G4 was to P4.
There are a few exceptions where Altivec truly shines (MP3 encoding, some type of video processing) where the Mac was proportionally faster, but inevitably slower just because the PC MHZ was so fast. But again, in general, I found Altivec to account for only 7% speed increase proportionally on average.
I have not kept up with the data since the G5s hit, so I can't comment on them.
I did all sorts of tests. 3D renders, Photoshop (dozens of filters and processes), text processing, file processing, I/O file swaps, network performance, etc.
My data concluded very convincingly that the MHZ myth is not a myth at all, but indeed fact. More or less.
Obviously there are exceptions, as I mentioned above. But a year ago those exceptions didn't mean much, because the speed gap was so huge, it didn't matter if the Mac was proportionally 50% faster, because it was 200% slower in actual MHZ netting -150%. And of course you can skew the data by starving a box for RAM or using slow drives or an outdated ethernet card, etc. But, IN GENERAL [he says one more time], similar machines performed proportional to their MHZ. Now I suspect that the G5 may change that a bit given its amazing architecture and solid chip. From what I've seen playing around at the store, the thing is just freaking fast. It seems to perform about 30% greater proportionally to a PC MHZ rating. That's just my semi-educated guess after comparing some charts and such.
Fortunately, those days [of slow Macs and huge speed gaps] are over. The speed gap has closed or narrowed to the point where it's not a big separation. I don't keep track of the data anymore because I love and prefer my Mac for reasons beyond pure horsepower. And when I grab a G5 this fall, I guess it will be the best of both worlds.