Not sure about getting gears, b/c I drive on the highway alot. My pop just got 3.73, and he told me about his RPM numbers. I like being able to cruise @ 80mph under 3K.
Yep, you can easily cruise under 3K at 80MPH with the big-time overdrive 5th gear and 3.73s. Get the gears, best bang for the buck. The car will feel like it lost about 800 lbs.
You will notice gears the most for the amount of money you have to spend. You can also go with a timing adjuster which will give you a little more power. Lowering springs are a nice appearance and handling mod.
If I were you, I would put that $400 in savings toward a blower, though. I think everyone's first mod should be a blower, so I might be a little biased.