Best/cheapest battery+isolator I can put in my trunk for dual batt setup.. Have optima yellow under the hood.?

Want the most power I can get and have $250 to spend on a bettery.. And also what type of isolator shuld I get (like how many amps rating should it be) and can I get away with maybe getting one of the cheaper isolators I saw at a local Pepboys (not sure of the brand but think it was like 40bucks) Please help with links/ideas/brands/prices if possible.. Thanx so much in advance.