more ram memory

I am looking to add more ram to my computer I already have 1g of pc2 4200 and was looking for the same but havent found any at local stores but I have found Optima 2GB PC2-6400 DDR2 DIMM 240 pin like what I have and it says it is backwards compatible to pc2 4200 I know that means it will run at 4200 not 6400 but would it be better to remove the 4200 and leave the 2 gigs of 6400 or 3 gigs of 4200 , sorry for the long post

 

More slow memory almost always outperforms a smaller amount of faster memory. That said, a very small amount of motherboards will give you issues if you mix speeds. It is usually recommended to play it safe and match what you have in there already.