One other suggestion for you, in addition to the great responses given already: Go to
Sharky Extreme and look at their current gaming systems. They do a budget system that stays under $1000, and they do an extreme gaming machine that has a $5000 limit. I just built a new core system (CPU, motherboard, RAM) using a few of their suggestions. Here's what I just put together about 3 hours ago:
-------- New parts -------
MSI K8T Neo 2 FIR
AMD Athlon 64 3000+
Corsair 2x512 PC3200 400mHz DDR
-------- from previous setup --------
GeForce FX 5800 Ultra 128MB
120GB Western Digital Caviar 7200rpm drive
all other misc. hardware...