I'd use the speakers you have up front, the 6.5's in the rear deck, a decent sub in the trunk and a 5 channel amp
http://www.bostonacoustics.com/GT-50-Refurbished-P713.aspx
Boston Acoustics GTA-400m Mono subwoofer amplifier 400 watts RMS x 1 at 2 ohms at Crutchfield.com
boston acoustics gta-400m on sale at crutchfield.com for $149.99, free shipping and the best return policy if you don't like.
Maybe I should've been more specific, "I go with what I know" means I don't know much about ARC, so ARC is unknown to me personally, ARC could be better than JL.
I would go with the JL, reason? I go with what I know, I don't like spending my money "hoping" the unknown is good. I know JL is good, have had and do have their products installed in my rides, never a problem.
you have two options here, 1 sub, or 3 subs, and I've never heard a amp that was 10 cents per watt that was worth listening to: 500 watt amp for $50=junk. Infinity KappaFive is a decent amp. look here for enclosures / subs that will work... Chevrolet Camaro Stealthbox by JL Audio