The system in my sig cost $1300 with overnight shipping nearly 1 year ago. The same or comparable parts could probably be had for less than $1000 these days. The only exception is that I would not recommend getting the 8800GTX. It's a fantastic graphics card but I believe they've stopped making them and the 8800GT and GTS and 9800 GPUs are cheaper, cooler and perform the same or a little bit better except at very high resolutions. Overclocking is very easy and the payoff can be huge so I'd get some decent cooling.
If OSX is your thing you'll be very pleased. A quad core Mac Pro would cost, at a bare minimum, $2300.
Here are the results of a current quad core Mac Pro in Geekbench.
Here are the results of my $1000 quad core in Geekbench. Another user on this forum has a similar quad core Hack that performs even better. Photoshop performance is excellent as is 3D application performance and rendering.
The disadvantages would be that you don't have any outside support for your computer. You have to wait for the OSX86 community to patch the patches so you can update (usually takes a matter of days, if that long). You can't add another core. That's probably the biggest disadvantage. But, if you never planned on spending that much money in the first place this is sort of a non-issue. If you do want 8 cores you could always go with an Intel server board and 2 2.33GHz Xeons which will all overclock to 2.8GHz with a very simple operation. If you did that you'd come out way ahead of Apples offering as far as price goes but get the same performance. If you bought a Skulltrail motherboard you could overclock those things even higher for some amazing performance.
For example:
This system will overclock to 2.8GHz easily and it costs less than a current quad core Mac Pro 2.8GHz and the Mac would have 4 less cores, 6GB less ram, a worse video card, and 700GB less hard drive space. If you match those exact specs on Apples web site you come out with $4749. You could buy the bare bones Mac 8 core for $2700 but if cost is an issue, by the time you match the Newegg specs you'll be well over $3000 if you upgrade the hard drive, video card, and RAM. Also, the Newegg system has a Bluray player/burner. It wouldn't be much use to you in OSX but if you dual boot with Vista it'll come in handy.