apollo are these the factory cast items on your camry? if they are i think you might be better off buying a set of extractors as i dont think there would be much point in modifying standard headers.
from memory 4-1 give more top end power 4-2-1s give more bottom end so you need to decide what out want out of it. personly i think just about every 4 cyl N/A car i have driven would benifit from more bottom end