I'd sure like to build a $6000 computer, even if it's not mine
anyway, what people have suggested is good so far.
a 6800 GT is a very good card, which you can overclock past Ultra speeds.
just don't get an XFX or a Chaintech, they do not make very good quality cards
the DFI NF4 Ultra-D is probabbly the best overclocking board you can get. on top of that it is very good quality
for a sound card, use the one that comes with the board. it's 8 channel (7.1) which means you can use surround sound speakers.
for a CPU, definately either a Venice or San Diego core Athlon 64.
the Venice and San Diego are both exactly the same, except the san Diego has 1MB L2 cache instead of 512kb
your CPU's cache is used to fill up with instructions, so the CPU can process them a lot faster than if they had to retrieve them directly from RAM.
it also helps because RAM has to 'refresh' all the time.
RAM is made with millions of capacitors, which basically store charge according to what bit they are holding (a 1 or 0)
now, because these capacitors are so small, they don't hold their charge very long (we're talking microseconds)
so RAM has to refresh to keep the charge in the capacitors
every time RAM refreshes, the CPU used to have to wait for it to finish before taking data to process. now with the cache filling up all the time, a CPU can process data easily