Good decision on the video card change. You will love that card.
Also, you do not need a 1000W PSU. Even to crossfire (2) 5870's. At MOST a 5870 CF setup will consume about 650W at max load (for the full system). To leave a little headroom a 850W would do the job fine. No need for a 1000W beast. It's a waste. You could probably get away with a SOLID 750W but a 850 is a safer bet. But do you REALLY think you will CF? or just sell it and get the next best thing at that time. That is what I always end up doing. I tell myself ill add a 2nd one but never did, up until last time I tried (2) 4870s and had TONS of heat issues. I ended up selling them and getting a 5870. So lets get real here... do you actually have plans to crossfire? If not get a solid 600W and save yourself come bread. Also, Silverstone would not be my brand of choice.