Will you be doing any towing? If so, how much towing?
Usually twins provide the best of both worlds, towing and out and out performance but they cost quite a bit more.
A big single works well for realitively cheaply compared to twins, but there are difinete tradoffs.
Do you have a automatic transmission?.
If you do a search for 'best+turbo+upgrade" you'll find some good stuff, but you have to be more specific.
Oops, edited after noticing you did put year in.
IMHO, for me, a HTT 62-14 with the extended tip "killer wheel" is probably going to be my next turbo, but it might not be the best for other people. But I'm going to take care of my stock transmission first, and I believe a lot of folks here might say the same after you get a good programmer or hardware box like TST, because you are living on borrowed time otherwise.
Probably turbo wouldn't be the best upgrade you could make right off the bat.
A bigger turbo alone just gets you more air-useless by itself unless you put more fuel in via programming or hardware (an example of each would be Smarty and TST box). It doesn't take much past stock power to toast your transmission, so it's saving money to address this next. A transmission upgrade all by itself can improve 1/4 mile ET's by probably 2/10ths, easy (were talking auto here)
You have air intakes, and exhaust (right about '03 or '04 you had a factory upgrade of a 3-1/2 downpipe to a 4" downpipe, if the latter you can replace the muffler and gain some HP, if it's the former, it would help to get a bigger 4" exhaust along with a better muffler)