You really have to ask yourself how you intend to use your jeep. The four cylinder is great for off road IF YOU KEEP THE SUSPENSION STOCK. If you lift it or go with bigger tires, regear as was mentioned. BUT, in all reality, if you want more power then sell what you have and buy one with the six. You will probably only be satisfied that way.
If you want to swap in another engine in your existing jeep, it can be a pain in the butt. I put a V6 in my and if you look at my "signature" it will show you where to go to look at it. It is so much easier buying a jeep already set up with what you want. Then when you are ready, you can just rebuild what you have when it gets low on power through high mileage.
Swapping in another engine for your 2.5 will require a new transmission and bellhousing, clutch, modified driveshafts and electrical system modification to match up the new engine to the old electrical harness. Computer and shifter will have to be changed also. You can do it but it is simpler just buying what you want already and treating her right.