EDIT: Holy ****, user above me necrobumped this....
Let's see...Early 2011 cMBP i5 2.3 dual, 384MB HD3000. I play with Fancy Graphics, 8 chunk loading (16 on 1.8), and turning off just about everything. Minimum smooth lighting vanilla, about 35% on optifine. I play with a decent amount of mob farms around, both manual and automatic.
1.8 Snapshot 14w30c - gameplay, average 20fps on world load, during chunk loading, 35fps gameplay. No optifine for
1.7.9 Non-Optifine - 30-40fps consistently, depending on how many chunk loaders I have active/mob farms
1.7.4 Optifine Vanilla - 60-80 fps.
1.7.4 Vanilla - 30 fps
1.7.4 Optifine Bukkit - 20-40 fps (server)
1.7.2 Optifine Vanilla - 50 fps
1.7.2 Optifine Bukkit - 20-30 fps
1.7.2 Vanilla - 40 fps
1.6.4 Optifine vanilla - 40 fps
Honestly minecraft isn't something you have to worry about, assuming single player. Multiplayer with bukkit will heat it right up if there are lots of plugins installed. Don't even think about ftb/crackpack/etc, lol.