I think that optifine is not needed these days in 1.6.4 .
But you will have problem if you have 32bit os and less than 4gb ram.( uhh)
In 1.4.x era fps was problem but not ram.
After 1.4.x FTB Team decided to add more mods to their modpacks which causes high ram usage :( .
So if you want smooth...