I already have a graphics card:
Gigabyte HD5770 1G Super OC GDDR5 PCIE2.0 DVI HDMI
My current system runs on a Corsair HX 520W. Absolutely brilliant and I've never had any issues with it. I'm wondering if I need to upgrade with this system or if its safe to go with my old one? I used an online calculator and it came out at ~400W with dual HD5770 in Xfire (which I may do later). My thoughts for an upgrade would be:
Your excellent Corsair HX 520W will be fine with your HD 5770 and the new parts and also work well with any single graphics card you'd probably want for a future upgrade, or the 2x HD 5770 option you mentioned.
Your parts selection is looking excellent.
What's the resolution on the monitor you're using?