First of all, keep this in mind: Minecraft is written all in Java, which requires you to have an over avarage to be able to do anything fancy with it. The thing is, first of all, Java requires more CPU than GPU, something I myself noticed when I upgraded my GPU from a 660 2GB to a 7970 3GB, and didn't get a single FPS boost. That would mean that you would need to upgrade your CPU if anything as far as I know, or overclock it (like me xD), which actually DID give me a frame boost. Anyway, keep in mind, most monitors only output 60FPS, so you usually wont need more than that, personally I like to limit mine to 70FPS as it feels a bit more responsive with the mouse.
Conclusion is:
CPU is the main issue as far as I know. Upgrade or overclock.
Consider if you actually need 100fps+, I personally don't find it any better to play with than 70.
Ask Notch to rewrite Minecraft into something else than Java
#BlameJava