If you are looking to use high power on a budget check out the new db-500 from TC Sounds, 20mm Xmax, high power handling (recommend 200w-700w RMS), Low Fs, in a .6 cuft box for $100. You can find out more if you google around. I own a TC sounds driver and the quality is top notch. This looks like a very good bang for the buck driver.
Never heard of 'em you say? Sure you have, TC Sounds supplies subs for Eclipse, Elemental Designs, Aerial Acoustics, Oz Audio, Styke Audio (until recently), SV Subwoofers (A.K.A. SVS, except for the PCi driver), and many others, and has recently started selling direct to the public.
I appologise if I sound like an advertisment, I am in no way connected to the company, I just love their product and thought I would pass on an interesting option.