I haven't really looked into super-expensive systems all too much, but I'll try to lend some advice.
I mean, it really depends on how frivolous you want to be. You could SLI two 8800 Ultras, but that would cost roughly $1000 in it's self.
I agree with g-paw on the mobo and cpu; however, you could wait a little bit('till 1st Quarter 2008) and go for one of those new fangled peryn's(spelling). For your GPU, you could go for an 8800 Ultra, 2900, or wait until ~mid November to see the new cards that are coming out.
On a high-end system you will obviously want a tier 1 or 2 psu, with significant wattage. It really depends what you are putting in it though.
The case is really personal preference, but you have to make sure that you have adequate cooling/airflow for that beast that you are making. You could probably get away with a fairly pricey one.
Then there are a bunch of frivolous accessories that you could add.
-Sound Card
-Super Huge Hard drives/Raid
-Huge LCD which is what you would want with a expensive video card.
I don't know, I just needed something to do for like 10 minutes and this sounded like a good idea. =\
Hope I helped.
-Adam