Comparing Apples with oranges @Marco Alfarrobinha 17:34 @AC 14:56
You are comparing Apples with oranges, apologies for making that figure of speech into a pun, but it's true.
Your comparison is purely based on internal hardware and benchmarks and not the OS, the software available or the experience of using the machine.
It really depends on what you want to do with the machine: if, like me, you want a portable means to learn Final Cut studio, one of the leading film editing systems, then you'd get a Mac as only it can run this software. And if, like me, you wanted to learn iPhone/iPod touch development (I only forked out for a iPod touch though), again you'd need a Mac, though platform restrictions appear to becoming more relaxed here.
However, if you want to only run Adobe software such as Premiere, or the leading After Effects (better even than Motion on the Mac, apparently) then I would say get a desktop PC for as you say much cheaper and with the saving get a companion laptop, still with some pennies left over. The Adobe license allows for 2 installs provided you are the only user, using one at a time. With this setup you get the total flexibility of building the desktop PC you want with a companion portable machine for on the move. Discussion I started about platform choice for Adobe, with some useful answers from more experienced users here: http://forums.adobe.com/message/3051534
My thoughts are based on that I own a similarly spec'd MacBookPro but from the June model update earlier this year and with faster 7200rpm hard drive, hoping to benefit from an eSATA drive via the express card interface later on when cash allows. I also own a desktop Windows 7 PC upgraded from a cheap Celeron Vista bought from a supermarket but now has Intel 1.8Ghz Core Duo with 2Gb RAM, fanless ATI graphics, near silent main fan. Cheap as chips for these parts off ebay, I would agree and runs sweetly too, plays 1080p HD video effortlessly.