EBITAD,
Are you having someone build the computer for you, or is this some kind of 'package' deal from a retailer? If you're not comfortable building a computer yourself (it's really not so bad), you may want to check out NCIX.com (or NCIXUS.com if you're in the states) because they will build a PC for you based on the specific parts you want if you pay the $25 fee for it to be done.
Personally, I would get a Q6600 instead of the E6850 and overclock it to 3.0Ghz because it is RIDICULOUSLY easy to do that with this processor. Regardless if you've ever overclocked anything before, you should read a quick a guide on overclocking for your motherboard on where the FSB (front side bus) settings are in BIOS because literally all you need to do is adjust it from 1066MHz (standard Q6600 speed) to 1333MHz (new 'standard' FSB speed of P35, X38, etc motherboards). You don't need to play around with voltages or anything like that. What motherboard are you looking at getting? I'd recommend the Gigabyte P35-DS3L ($100), excellent board with all the basic stuff plus a few extras like FireWire, and even overlocks really well.
I would also take the Quad core because there are already games out right now that take advantage of quad cores and more are coming. For other games that don't make use of more than 2 cores, you wouldnt notice any difference between the Quad Core and the Dual Core anyway - nobody can actually notice a difference between playing a game at 110fps vs 85fps
That graphics card is pretty much the best out there right now, but if you want to save a few (hundred) bucks, look at 8800 GT 512MB or the new 8800 GTS 512MB. Personally, I like ATI's 3870 512MB cards for their CrossFire options, but in terms of single-card solutions the nVidia ones are the way to go. Another thing to consider here is your monitor and what resolution you play at. I would say that a GTX is overkill for anything less than 1900x1200, any of the 512MB cards I mentioned should play games very well at 1600 or 1280 resoltions.
Also, don't skimp out on a Power Supply Unit, it's the most important part of a system! Brands like Seasonic, Silverstone, Corsair, and some Antecs are the way to go. With a GTX in there, I wouldn't get anything less than 500W. If I were buying right now, I'd get the Antec Neo Blue 650W PSU because it's a great value and that way the system should be running under 70% load, which really extends the life of the PSU and makes it operate much more efficiently (yay electrity bills).
Edit: Almost forgot, if you really want to get 3GB of RAM instead of 4GB, make sure you get 2 sticks of 1GB, and 2 sticks of 512MB. This will keep your memory running in dual channel mode, which is what you want. That said, there's some good prices out there on 2x2 GB (4GB) RAM, but then you would need Vista 64 bit to make use of it.
Well that about does it for my lunch break, hope that helps!
