Is this a good build?

I could't get all your links to work, so you need to check them. If you are planning on using a 550 watt power supply with that video card that is listed, you are in for some heartaches. You need to prepare for the future and I suggest that you start looking for a quality 750 watt power supply to allow some head room. Even with the single EVGA 9800 GX2 you have listed, I would not try that baby without a minimum of a 650 watt power supply by a major manufacturer. Here are some great power supplies you need to consider and think big so you don't box yourself in with a bare minimum power supply:

Seasonic - any model
Corsair - any model
Thermaltake - "Toughpower" series only
Coolermaster - "Real Power Pro" series only
PC Power & Cooling - any model
Silverstone - any model

TBH, I think a 550 watt can handle that type of build...if it is of good quality...This one will drive a single 9800GX 2

http://www.newegg.com/Product/Product.aspx?Item=N82E16817151027

I was going to get a 900 watt ABS, is this a good psu?

http://www.newegg.com/Product/Product.aspx?Item=N82E16817153062

An ABS is not worth buying if that is the name of it. Frankly, I never heard of that name, so could NEVER recommend an off brand for an excellent video card.

However, the link you listed is the Thermaltake "Toughpower" 850. That is a top notch power supply and you won't regret it if that is what you are looking at buying and it will not only do what you want, but allow some room for growth as new and better video cards come on the market.