It seems to be a branded 620 watt power supply and would probably be fine but personally I'd like to up it more to allow for adding extra components.
I do notice that they seem to only have the choice of the two. Mind you I suppose you could buy it and then replace it though it is a bit of a waste. Would phoning them or emailing them be an option.
I did a quick check on the PSU and it seems to need a 900 watt, surely thats not right?
I used this click here
Hopefully someone else will comment as that can't be right, you add a second PCI E graphics card and the thing jumps from 600 watts to 900 watts. Thats definatly not right. I think that program has to be mucked up.