Well then VX450W would do just fine. Just asked about your future upgrades so that you can get a bigger PSU and wont have to change your PSU in the future when you upgrade. But since you wont, then VX450W is ideal.
Just curious, have u thought about adding a second 5770 in X-fire as opposed to going with the 6870? The performance should be very close, depending on individual titles X-fire scaling, but the dual 5770 route would be approximately $100 cheaper.