Yeah, it's not the frame rate per se as it is the fact that it will suddenly jump around between 4 - 10 fps depending on what's going on. I honestly think my CPU is bottlenecking things the most.
With respect to addons, it's been said that Blizzard's Lua implementation is actually quite efficient, but you will of course have issues with poorly optimized addon code. That and "kitchen sink" ones like xPerl.
Unfortunately, I'm just not in a financial position to buy a new computer now, for various reasons. The situation may resolve itself by next year... I hope.