[Solved] $1000 Gaming PC Build Advice

Hello,
I'm looking to build my first desktop computer (technically my 2nd but I had help from a friend on the first one) and have been looking at parts for the past week but am ready to admit that I'm not knowledgeable enough to get the best bang for my buck.
My current gaming machine is an XPS M1710 with
Windows XP Home Edition
Intel® CoreTM 2 Duo Processor T7600 2.33 GHz
4GB Dual Channel 667MHz DDR2 SDRAM
200GB Harddrive at 7200 RPM
DVD+/-RW DVD Burner drive
BLUETOOTH enabled
nVidia GeForce Go 7950 GTX with 512MB Video
I'm hoping to sell this and then add a little of my own money to bring out a gaming desktop. My laptop can play Assassin's Creed at 1920x1080 while in the city but I have to tune it down while in the Kingdom. I can also play Creed at Medium settings alright. I'd like to be able to blow these out of the water as well as be able to handle new games coming out such as Assassin's Creed 2 and other FPS games/ racing games.
My first thoughts were to SLI 2 nVidia GTX 275's and find an appropriate motherboard/ AMD CPU such as the Phenom II X4, but I'm no longer certain of what would be best.
I'll start off with being limited to a 17" flat panel screen that I don't know the specs of, but I'm hoping to eventually hook up to a Sharp Aquos 40" at 1080p with a DVI/ HDMI cable.
So, for the record, I have approximately $800-$1200 for a gaming machine, give or take a couple hundred--depending on how much more performance I could squeeze out for the higher price. I'll keep the case, keyboard, mouse, speakers, bluetooth, etc cost separate.

So, I'm looking for advice on CPU, mobo, GPU, power supply, memory, hard drive, OS etc.

APPROXIMATE PURCHASE DATE: Within the month
BUDGET RANGE: $800-1200 or close to that

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Gaming, making backups of my DVDs, web surfing

PARTS NOT REQUIRED: keyboard, mouse, monitor, speakers, case, bluetooth

PREFERRED WEBSITE(S) FOR PARTS: newegg.com or tigerdirect

PARTS PREFERENCES: Friends supposedly say lean toward nVidia GPU and AMD CPU but I'm flexible

OVERCLOCKING: New to it, but am ready/ willing to learn
SLI OR CROSSFIRE: Definitely an option, if it's easy enough

MONITOR RESOLUTION: Eventually a 1080p 40" Sharp Aquos TV

ADDITIONAL COMMENTS: Best bang for buck is preferable, and I can learn new technologies/ techniques of need be, adaptability to new upcoming games

Thanks for the post hunter315!

I was looking at either
http://www.newegg.com/Product/Prod [...] 6811103014

http://www.newegg.com/Product/Prod [...] 6811146028
or
http://www.newegg.com/Product/Prod [...] 6811208005

for cases. I know some are a little pricey but they caught my eye, got decent reviews, and seem to be well circulated.

And the AMD processor is certainly not a limitation, if it's a better combination to get nVidia with Intel over Radeon with AMD then I have no problem with that switch.

Sounds good, that's the case I was favoring anyway.
And alrighty then, we'll go with the AMD/ Radeon

a 4890 should be able to handle it.

Oh, and if need be, I do have a Windows XP install disk, if I should just use that instead of buying Vista, I'm not sure.
And would there be much of a benefit to getting the Asus Crosshair III instead, or is that just a preference thing? I looked on some other forums and there seems to be substantial arguments for both sides...