No the storage of an SSD does not matter , in terms of speed you should look for read and write speeds on the sites you're looking to buy it. Faster read speeds will help booting , running programs , loading them and the write speed will help when you're moving different files.
Adding 4 GB's of RAM will slightly improve your computer/gaming performance by a small margin.
You can find 64 GBs SSD's , but you should buy a 128 GB one.