You need a Mono amp that puts out at least 800W RMS.
If i were you i would go with one of the amps on this page with 800W RMS (Infinity Kappa, Polk Audio, MTX Jackhammer, MTX Thunder): http://www.crutchfield.com/g_120/Amplifiers.html?pg=2&showAll=N&tp=115&nvpair=AG_Number_of_Channels|FFMono_Subwoofer&nvpair=FFRMS_Power@@Channel|[rank7]351+and+up
Also to get a 2 ohm load wire it like in this diagram: http://akamaipix.crutchfield.com/ca/learningcenter/car/subwoofer_wiring/2DVC_2-ohm_mono.jpg