A new CPU won't help you without a new GPU to go with it. You could do a major upgrade, but it'd be almost ground-up. You could wait a while, as cumi suggests, or do a build now. You'd have to go over $1000 to make it worth it, or a little less with salvaged parts (case, HDD, PSU if it's good enough, ODD).
An SSD would have a nice, tangible benefit now.