Definitely get you a GPU. You can probably find an old 5850 on Craigslist or eBay for under $100.
Of course, if you don't have a PC with a PCI-E slot it will do you no good
I'm working on convincing my wife to let me expand my rig, but apparently things like mortgage payments are more important...
I have two PCI-E slots free and i can probably free up a third or replace the NVidia card I have that is slower than CPU mining for scrypt to free up the fourth, but this Mac Pro sucks up a lot of electricity just sitting here. So I'm looking at building a dedicated rig for scrypt mining. Right now all I have is FPGA and I have a BeagleBone Black on order so that I can run the FPGA without the desktop being on 24/7.
If I eliminate this electricity-guzzler from my power bill, I'll get my electricity down to 10.5¢/KWh from 19.5¢/KWh, and that will make a difference regarding what kinds of hardware I can add and be cost-effective.
But yeah, maybe I ought to just grab one of those 5850s just to have it set up. I can always swap it out into the dedicated rig once I get one.