you cant be running in dual channel mode if you have two mismatched sticks of ram
you can download cpu-z if you want to (free) then go to the memory tab from within the cpu-z program and look in the uppr right side of the report >>>> it should say single channel mode which is not as good with performance as dual channel mode
dual channel needs two identical sticks to make a matched pair
like 2 x one gig sticks >>>> then sell of the old 512 on ebay
make sure you buy the exact same stick of one gig as you already have
http://www.cpuid.com/cpuz.php