For mining Bitcoins I would recommend ASIC hardware. You return for investment will be a minimum of one year and will take longer as the difficulty increases.
What about going with a Avalon chipset? What I have been reading is that ASIC would take much longer. I was debating getting a BFL, but haven't read many good things about them on the forum.
If you're getting the Avalon chipset you'll also need to build or purchase a board to go with it. Not recommended unless you know what you are doing.