That PSU is overkill. A single HD6850 does not use a lot of power. Any of the 80+ bronze Antec Earthwatts PSUs can run your system, even the 380W model (but I think there's a good price on the 430W right now). Use some of the remaining money in your budget to add another RAM stick so you can run in dual-channel mode.
Edit: Make sure it is an IDENTICAL RAM stick.