1)AMD's tend to run much hotter than intel's.
This was true maybe 3 years ago, no longer the case. Intels run much hotter than AMDs now
3)Get plenty of FAST ram, meaning the fastest your PC can handle, usually DDR400. Dont settle for the cheaper DDR333 and all that stuff.
DDR400 is old news, any newer system will run DDR2 at 667 mhz in dual channel mode (effectivly making it 1.3 ghz). Look for quality ram for sure (I'm a fan of Corsair)
6)Dont be afraid to spend $$$ on a good power supply, get something BIG.
VERY IMPORTANT. A psu can make or break a system, don't go cheapy. Compare power outputs at temperature (they are usually rated at room temp, they dont stay that way at operating temp) also look for ripple. I just replaced my PSU with a Seasonic 600w and am very happy with it (very quiet) but at least spend over a $100.
Power supplies are the most failure prone part of a PC so do your homework
Beyond that, look at benchmarking sites for the DAW you are looking at using. Sonar will run nativly on win64 (the only one AFAIK) so that would be a logical fit if you go AMD x2 and Win64.
Cubase runs best on AMDx2 processers (as far as track count, CPU load, etc) and is at par with Quad G5s as far as performance.
The bottom line is that all the top of the line CPUs will be withing 5% of each other so it is up to you what you want to run. But the most important thing is to really do your research, find the mainboard you want, look it up in as many places as you can then go buy it based on what you find.
As far as buying a Mac now for a DAW, be warned that PPC is history in the mac and Steve Jobs himself declared that they will be an all intel shop in about a year so you are left with this quandry: buy a PPC based mac and have all your software updates dry up in a year for your software (which may or may not be acceptable) or by an Intel based Mac now and wait for x86 native Mac software. I find neither option acceptable and is one of the reasons I dont get near Macs
Personally, if I had to reccomend a CPU right now, I would go with an AMD X2. The multi core is the future for sure, so theres no reason to low ball yourself