Unless you have a system with a BIOS that you can set memory timings and speed, the motherboard will run the memory at whatever speed it wants.
Most of the time you will get best memory performance with a 1:1 mem/(BCLK/FSB) ratio. You can plug in faster RAM, but chances are if your current RAM is already running 1:1, then that is as good as it will get. If it is a lappy, then your options are even more limited.
Generally, 3gigs of RAM will get you by OK even on VISTA. With 2 slots, 4gig (2X2gig) is the usual configuration. Whether that 1 additional gig will make enough of a difference to justify the cost is the question.