Four things.
1 - Budget - Just how much do you want to drop on this bad boy?
2 - Use - Are we talking hardcore gaming, video editing or simply just crusing the net and office?
3 - Overclocking - Are you technically savy enough? Think of this as a JB3 for a PC - are you comfortable uping the performance above stock to make serious gains?
4 - Timing - When do you "need" the system? Can you wait or is it a must right now?
The timing question is pointed at this - Intel's current i7's are great CPUs - HOWEVER - the next itteration of i7's, which should be due soon, will improve more so and with Intels pricing policy, won't cost much (if at all) more than the current i7's.
If it were me I'd go the following route:
CPU = Intel i7 920 (arround $200 right now @ Microcenter)
MB = ASUS
(this) is a good start.
RAM = 6 gigs minimum - depending if you overclock depends if you'll need the faster timing stuff.
HD = Get one of these,
Western Digital 1T Black - fast, reliable.
VID Card = right now, I'd say get one of these
ATI 4890 - if you need more horsepower down the road, buy a second and run them in Crossfire, it'll destroy anything out there right now and it's sub $200. Brand wise, XFX is lifetime waranty and have the best customer service.
CASE = This is easy -
Antec 900 - shop around, you can find these sometimes as low as $50 shipped at Fry's. Great air flow and easy to work with.
PSU = DO NOT SKIMP ON THIS! Your PC is only as good as your CPU. 750W minimum I'd say so as to future proof if you get another Vid card -
this -
this and
this are good options.
HEATSINK = You'll NEED a better HEATSINK than the Intel Stock - its like an intercooler for your CPU -
this or
this paired with
this and you're golden.
MONITOR -
This is NICE - sure there are better out there, but for the money this 28-inch is hard to beat.
Best of luck with the build.