Need advice for gaming+3D anim system ~$600-900, monitor extra

Could some expert builders please share some advice on this system I am dreaming of?

I'm looking to build a system for weekend gaming and playing around with Maya/ZBrush/UnrealEd3. I'd like to be able to play games like Modern Warfare 2 and Crysis on full settings. Also, how much money is actually saved by building a DIY 600-900$ gaming system versus buying a pre-built gaming system from a retailer?

APPROXIMATE PURCHASE DATE: Would like now, but can wait. Is there any price advantage to buying parts before or after Christmas? What about the Windows 7 + DirectX11 factor and how those new standards are changing hardware?

BUDGET RANGE: 600-900 or so. Monitor priced separately.

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: gaming, gaming, gaming! And messing around with Maya/ZBrush/UnrealEd3.

PARTS NOT REQUIRED: keyboard, mouse, speakers, OS, optical drive, sound card. I have an 19" LCD from 2004 also I could use.

PREFERRED WEBSITE(S) FOR PARTS: I guess newegg and tigerdirect are the big ones? Should I be looking elsewhere also? I just want a reputable dealer with a good return policy.

COUNTRY OF ORIGIN: USA

PARTS PREFERENCES: Anything reliable but not overpriced.

OVERCLOCKING: Only if it doesn't void the warranty on suggested parts!

SLI OR CROSSFIRE: I'd like that capability if it gives good performance for the price.

MONITOR RESOLUTION: I currently have a Dell 193FP 2004 1280x1024 19" LCD with VGA-only input. Can you suggest any great deals on a nicer monitor with better response time for gaming? Is widescreen the way to go with gaming these days?

ADDITIONAL COMMENTS: Looking for a modest, sleek case that is big and easy to build in. Quiet is preferred, but some noise is ok since I'll be gaming with headphones.

Also, I'll probably only have time to game on the weekends due to work/family, so I'm a little reserved about buying "too much" computer because it will just sit there getting outdated during the work week.

Thank you!

I don't currently have a preference...aren't ATI and nVidia kind of like Coke and Pepsi? I'll take whichever one will give the best gaming advantage for the dollar over the next few years (until it's time to upgrade!). So do you have any suggestions for a build? Thanks!