if you're fine with your current mobo, keep it, do a BIOS update, then upgrade the CPU, all in that order.http://www.asus.com/Motherboards/Intel_Socket_1155/P8H61M_LE/#CPUS
you'll need BIOS 4104 if you want a 3470.
from there, any cash you had planned on getting a new mobo can be re-allocated for a better GPU if you wish.
if you want more onboard USB 3.0, then a new mobo makes sense if you're willing to spend, but not a Sabertooth Z77 if you're going for an i5-3470. a B75/Z75/H77 will do you good for half to one third the price of the Sabertooth.
if you want to OC, then grab a 3570K and a decent Z77, say Asrock's Z77 Extreme4, unless the Sabertooth's build quality is a major turn on for you or you want to get into enthusiast level of OCing.
EDIT: Haswell will be using a new socket, so no, it's not backwards compatible.