Well, the dual-cat question vs. aftermarket depends on your particular state, and in CA it even depends on where in the state!
The OEM is a high-cost deal but here's what KC Converters quoted me (your mileage may vary):
Thank you for your inquiry, Todd.
Part #BN-522H and BN-530A or...
Depending on your state's emission laws you may have to stick with dual cats (like CA) or go through a painful referee process.
Check out http://www.kansascityconverters.com/ for a good alternative to new...
Saved me the hassle of the CA smog nazi system. The you can still install a cat-back...