Unsure of Ram Speed I can use??

I need to purchase 4 1 GB sticks of ram for a HP Pavilion PC Desktop Model #A1720N Product# RT641AA #ABA. The user guide says that the ram to update it with is PC2-4200 MB/sec 553 MHz 240 pin DDR2 SDRAM. What I am confused on is there is other PC2-6400 800 MHz ram for sale and it seems like as long as I bought and used two pairs so they are all the same exact stick of ram that the dual channel would work great and run even faster. I am unsure if it even matters to buy ram that has speeds a little faster but figured I’d see what you guys thought. Thanks!!