What do you use your computer for, and where do you find it lacking?
I use to play MMORPG online, but I don't anymore. Mostly, I surf, edit photos, watch and edit HD videos, listen to music, and occasionally, I'll play a 3D game.
The biggest bottleneck is transferring data or keeping multiple windows open while surfing, watching/editing videos, and listening to music.
I do have 3 TB of pics/movies/music/games on separate external hard drives that I like to move back and forth, so I've thought about upgrading to a USB 3.0 card or a motherboard with USB 3.0 and SATA-3 600, which I believe is faster than Firewire 800 and USB 2.0.