Finding a four stick memory configuration that will WORK is not as easy as it should be !!!!
You will need to check to make sure both of your MATCHED pairs of memory modules have been tested for your mobo.
This "checking" can be done by going to the memory makers website ( youre lucky you picked a good brand "Corsair") and use their memory selector or compatability chart. This chart will give you a memory model number which has been tested in a four stick configuration. You may have to sell off the ones you have if they have not been tested and guranteed in a four stick configuration. Makers such as Corsair, Crucial, OCZ gurantee if you buy models they have tested with your motherboard model they will swap, refund whatever until you have four working sticks of 512 memory !!!!
Gone are the good ole days when you can throw in any old memory. The recent mobo's are VERY fussy about ram. You can always sell off any un-needed memory on ebay !!
PS >>> sooner or later the Mad Dog 400 is going to bite you / I personally would upgrade to an eneramax or antec PSU of about 500 or 550 watts before it gets the chance to ruin some expensive computer components and then you will still have to buy a good PSU !!! You wont believe the stress a weak PSU can deliver to a system !!! not to mention the weird symptoms they can manifest themselves into
dont hesitate to ask more questions