Need help with Alienware PC

I think you'll be fine with the TX750 to be honest and I saw those at $109 before rebates etc
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139021 although for the extra few dollars I'd take the 850 for headroom
One thing you could do is put your build into http://www.thermaltake.outervision.com/ and you can see what you roughly need, and you can also alter it to include your intended new gfx card, see how much that alters things,
If you download cpu-z it can give you more info on your components, motherboard manufacturer/model included, its a handy thing to have
Moto