Considering power supplies degrade with time, and frankly I think it may be a little under powered for the build as is, I would definitely recommend upgrading to at least a 650 watt power supply. Try this one. Especially if you plan to upgrade in the future.
The recommended minimum psu for a 6870 is 500 watt , but they run fine on much less . You will be fine with the Antec . The likely total draw will be around 250 watts at peak .
There are AM3+ motherboards that cost only a little more . Search on newegg by price and the first 970 chipse board is only $85. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Since the motherboard is the hardest thing to replace you could consider saving money on the processor . A Phenom 840 ?