Holy crap those are crappy units for a modern system, that 700W unit has a 35A single rail, the corsair 450VX had 33A on its 12V rail, the reason your unit cannot keep up is because its probably rated about 200W too high, that 800W unit has at best 40A between its 12V rails, which is less than a good 550W unit has. Get the 800W unit so you have the most overhead, but my best suggestion would be get the new unit, ebay it and get a good unit.
Thanks for the advise. I am considering to buy a new smps. Which brand smps should I buy???
Corsair or Seasonic
Seasonic is cheaper than Corsair. I am thinking to buy 750 watt smps.
Any other recommendations???