as "soulrider" says, it's totally dependent upon what you are doing. There are a number of bottlenecks possible in a system, e.g. processor, memory, disk, graphics card etc. obviously you will speed you machine up best by upgrading the part that is slowing you down. Is that English?
To be honest, unless your system is primarily memory bound, I'm not sure that adding 64Mb to 96 is ging to make it "run very quickly". What operating system do you use? have a look at the system tools to display memory usage or get your hands on some tools that will tell you.
I know this all sounds vague, but for example if you are hoping to use the machine for games the video card will have a far greater effect on speed that memory. If you are wanting to run several applications at the same time, throw memory at it. At todays prices on memory though I would consider going higher than just an extra 64.
Important! have a look at your machine documentation to determine what combination of memory boards it supports if that is still the way that you want to go.
Final point repeated though. Do not expect that just adding another 64Mb will make it "very quick". It may improve performance, maybe even drastically, but only if you have a specific memorybottleneck such that an extra 64Mb will allow the machine to run from memory as opposed to paging out to disk.
