Header Help with (Bullets and Tune)

I have a '06 A6 car with Vararam and Bullets. I plan on adding headers and cats (and a tune) only. Should I go with 1 3/4" or 1 7/8"? I am running the Bullets with the PRT section. I would like to keep the PRT,but it doesn't seem possible with the longtubes. I wonder how much louder it will be???????? Any suggestions on headers and vendors. I was loooking at Chuck's COW mail order tune (it looks like he can get rid of the torque management). Thanks


