I reckon, at the moment, if you shop clever, you could build a monster PC for under a grand (Brit Money) or not much more depending on how much RAM. Raid system, SSD etc you go for. And that’s without skimping on anything.
Personally I stick with Intel processors and either Gigabyte or Asus Mother boards. Presently I have a 650 watt Corsair PSU, and trust me, my machine is full of stuff… like one 240 gig SSD, 3 standard hard drives, two UAD-2 cards, a PowerCore Firewire Element and a EMU 1820m Sound card.
It’s a Gigabyte EX58, I7, 6 gigs, fan-less NVIDIA Graphics. It does everything I want for now but I don’t use huge sample libraries. Sometime next year however I plan on building a beast machine with a fancy case and oversized cooling system for over-clocking (never done that before but, what the hell!)
I’ll be upping the PSU to about 850 watt and I’ll be sticking with intel, gigabyte or Asus but I’ll probably spend at least a month researching the rest, FireWire chip set, RAM etc etc.
Then, fingers crossed 
Edit: Also, my sound card is about ten years old and hasn’t been supported for years, yet it just keeps on working. Got all my hardware integrated into Cubase and everything. But alas, I reckon it’s days are numbered. Had my eye on the Steinberg Sound card for a while but I’m not sure about FireWire for sound cards. This new Tunderbolt technology looks interesting though, doesn’t it?