The amp you have now should be more than enough. Where do you have it grounded? Also how old is your battery?
2006 BMW 325i Titanium Silver
Mods- 19" AG M310's hypersilver, LCI xenons, drilled and slotted rotors, OEM rear spoiler, OEM exhaust tips, brushed black interior trim, Viper 5904 alarm + remote start
Audio upgrades- Alpine S920HD, JL XD600/6, Jehnert XE200, Morel Hybrid Ovation II, Audioque 2200D, 2 Alpine SWR 12d4's (forward facing sealed from trunk)