Funny thing about that. My crappy laptop(lucky to get 35 fps on absolute minimum settings) loses hardly any fps if I use a 128x texture pack.
I get 30 FPS with a 128x128... on fast graphics and a tiny render distance, and still get 0 FPS for about 2 seconds every now and then. Recently 128x128 packs caused all of the textures to become white (Indicating that my computer failed to render the detailed textures I believe) so I just stopped using 128x128 packs. Never tried 64x64 though, and don't plan to any time too soon.
EDIT: On another note, I just got my 100th post. Yay.