Here's my mod process with my car:
Catback, then did catless UP. I bought both used and spent a total of $160. You'll love the sound and the increased performance. I then did a DP, and then got an OpenSource tune. Overall, I spent $550 to go Stage 2, and it's completely worth it.
But since you have an 04, dont do any of this stuff until you are ready to get tuned. The 04's are far more finnicky without a tune than 02-03's. What I would do is accumulate the parts, and either you install them a day or so before getting tuned, or have your tuner do it the day of the tuning.