I wouldn't bother with a supercharger. You'd be better off to tear the engine apart and build it up to get decent boost to make the cost of the unit worth while. Paxton and Procharger are two that carry good units. I recommend ProCharger as they offer one of the better intercooled packages that should allow up to 5-6psi without doing much to the engine. Stock injectors can handle 6psi of boost as well, so no upgrade is required there. Expect to dish out $3-4K for a good kit.
Performance Parts Online for Cars and Trucks - Performance Center can give you some ideas
Gears are one of the better mods you can do for off the line performance. If you're comfortable with higher rpms on the highway, then gears is the way to go. It allows less gear hunting while driving, and can even hold an overdrive gear longer when climbing hills without overheating the transmission. If you have 3.55s or taller, I suggest going with at least a 4.10 for best all around performance. If you tow occasionally and again, don't mind higher highway rpms, go for 4.56 gears. This will also allow you to upgrade to larger tires without losing much performance...in the future.
Yukon Gear & Axle is one of the better places. Randy's Ring & Pinion is also a good place to get gears.