You can use 2 psu at once, one for the mainboard etc, and if u wanted to u can use the other one, hooked up outisde of the pc case, for the gfx card if it needs direct power to the card, or for the hdd and dvd to take the strain off the other psu.
well if that doesn't work then you can always buy some equipment which will allow you to use 2 psu's together. Don't know what you need or how much they would cost, I just saw some dudes use those when they needed more power for their oc.
But I'd just go with buying a 400W psu. It's quite a lot for a machine like yours but it won't have any negative effects, at least then you have room for upgrades.