I am going to have a cat back system put on my truck.It is a stock gmc sonoma with the 4.3 V6.I am going to do future modifications, but leave the engine stock.My question is would it be better to go with a 3" or a 2.5"?The other mods I will be doing down the road aways are electric fan/water pump,CAI,functional ram air hood and headers.I want the most performance possible, but don't want to hurt performance.Thanks.  

2.5...even that will probably be overkill.  

I wouldn't bother with a catback exausht system if your going to put headers on the truck later, then the catback would be useless, you would probably be better off getting headers first and then getting a set of header mufflers like, cherry bombs, hornies, or even dynomax, instead of spending money on a exuasht system that is eventually going to come off anyways.oh well, thats just my opinion, either way i know people who have done this and the results turned out very nice with a 2.5"  

Do they make shorty headers for a V-6? If so you won't be wasting money on the cat back system. Besides, if you have someone do the work they can not eliminate the cat system legally. At least that is what I have been told by the shop I take mine to. Even if they don't make shorty headers for the V-6 I would think that a good shop would be able to modify the system to work with the headers. If you do the work yourself you can get by with eliminating the cat, unless you live in a state that requires a yearly inspection of such items.  

By the way, the biggest I would go with as far as pipe size is the 2.5 inch. A 3 inch would not provide the back pressure it needs, some back pressure can be good.