I would recommend having 2 processors up. You will figure out soon who you want as primary and who you want as secondary. The 3 main ones people use are
Epoch/Paycom
www.epochsystems.com
Ibill
www.ibill.com
and CCBill
www.ccbill.com
Jettis
www.jettis.com is getting stronger lately. If you use 2 of those 4, you will be happy with the results.
ZoiNk