exactly what blue00gt said. to be honest though if you can't afford to rebuild your motor then i would wait until you can. then add a power adder. All power adders put alot more pressure on the bottom end. typically the pistons blow apart first. and imho 1 bad tank of gas could make your car ping enough to pop a piston. thats why most get forged stuff as it can take alot more abuse. Like blue 00gt said no more than 6-8 psi. and i would recommend a dyno tune! no mail order crap. good luck!
Leon
P.S. the problem with boost is, it is addicting. you start by saying oh 6psi is plenty and then before you know it your trying to run 20+ psi:D don't say i didn't warn you;)

OK a few things, the mach 1 and Gt DO have different pistons, if you are running a GT 2001 and new you must change the pistons and rod if you plan on running any more than 8 psi. In 01 Ford started using a softer material in the pistons and rods, which was causing a problem with cracking the pistons, a BIG problem. I am running an 8 psi 3 core ATI and when I bought it they told me that this was such a big problem they would not warranty anything on the kit if I ordered it for more than 8 psi. I personally love the centrifigal s/c. I can launch my car exactly like stock @ 1100rpm and it sticks and takes off like a bitch, I hit 8psi@ 2400 rpm and from then on its woooohhhhhh. I have not had any experience with twin screw chargers so I do really have much info, but I can imagine if you have instant boost then you are deffinately going to have a bit of a traction problem @ launch. A lot oh and the best price for the kit I got is on the web www.Superchargers4less.com the guys name is Mike. My kit is a stage 2 3 core intercooler on 8psi and it cost me a grand total of 3491.50 to my door; the only thing extra I had to buy was copper core plugs and 42 lb injectors from steeda for a grand total of 310.00 to my door. Hope this help some. And sorry for the long ass post.