You probably only need to plug in four of the eight pins. The mobo manual will say which ones.
Your PSU should be sufficient, although it is an older, inefficient design. The giveaway is the little red voltage switch. To my mind, those are actually little red warning flags not to buy them; you already have it though.
well isnt it all drawing power form the same place anyways? and that 4 pin is the normal 4pic cpu connector, it just makes it a 8 pin.
Check the pin-out of the connector... You are correct, in that the power all comes from the same place, problem is the difference between your molex and an 8 pin molex are quite different. Gkay's solution is a lot cheaper than buying a new PSU, and should work.
Personally, I'd get a new PSU. Antec makes some good units in the 650W range, as does Corsair, either would be a good unit at a fairly reasonable cost.
Your current PSU is probably enough, if it's not, go for the Corsair VX450W, no need for 650W with your system.
+1, 500 watts should be more than plenty. in general people buy psus that are way overkill for their system. it seems the thought that if some is good more is better prevails over buying a properly sizes psu.