If your fps sucks, and i mean like, really SUCKS, and if you have the launcher .jar, I can help. This is definitely a project, because it helps a lot of mac 10.4.11/linux users.
1. Open Terminal
2.Type in java -jar -Xincgc -Xmx(Amount of RAM, M for Megabytes, G for Gigabytes: Ex: 2G for 2 GB (Drag launcher jar here)
2a. YOU CH
3. VOILA! Your minecraft runs on (Your specified number here) of RAM for minecraft, building up as it goes.
3a. This will also enable you computer to have it's share of ram.

Hope this helps you users of those kinds of computers!!! :D