1500mb memory works slower than 2gig because it's 500mb less :s
1500mb works like 1500mb, not slower of faster.
What you probably mean is that you need 2 of the same frequency modules.
They don't need to be exact, but it's recommended you don't use any modules that have a higher frequency than your lowest one. f.e. if you use 1 512mb DDR2 533mhz module and 1 512mb DDR2 800Mhz, that 800 Mhz will only work as fast as 533Mhz, it will automatically adapt to your lowest frequency memory clock.
This MAY cause issues, but its rare. Only issues i can come up with is that if you force your ramclock to a higher setting than your lowest clock in the BIOS, it will start giving clonflicts.
