I never recommend Intel.  AMD paired with a good AATI video card works better, is no hassle whatsoever and costs a bit less.  Intels are just not worth what you pay for them.  And the 790GX series chipsets are great.

Quote - I've always built my own computers, getting parts from ncix.com (they also sell to the States, I'm very close to Vancouver BC, Canada where they are located).

Quad core, go with Intel motherboard (less hassles with Intel motherboards with graphics as compared to nvidia chipped motherboards, I've had both and my current Intel has had the least errors).

Also if you are on a budget, go with the LGA775 processors, i7's are still pricey. and the 775's are cheaper with Ram costs.
Go with the new  Win7 (and you can downgrade to XP Pro 64bit by calling Microsoft for free, if there are concerns).
I always salvage as much stuff from my older computer if I can, buying a new HD (Hard Drive) and using the older HD as my scratch-disk, SATA's can work spread over several HD's.
Video? I prefer ATI as (in my own opinion) it provides better looking AA results.

Everyone will have their own preferences (nvidia vs ATI, Intel vs AMD) I just prefer the above, as I've had computers with both, so far my own 775/Intel/ATI combo had been friendly with Carrara/Hexagon, and all the rest of my software in my sig.