You use 1 and 3 because the nForce 420 chipset has 2 independant memory controllers. Slot 1 is on one controller, and 2 and 3 are on the other. MSI recommends using slots 1 and 3 for optimal performance. Having 2 memory controllers means double the memory bandwidth. This only gives a small performance boost to most applications, but the onboard GF 2 MX is much faster with 2 sticks of ram present. The harddrive also benchmarks somewhat higher with 2 sticks in; my guess is Ultra DMA [direct memory access] likes the extra memory bandwidth.
My cost on which part of the system? The board (w/ shipping) cost me $205.
