Bang,
To get the best bang for your buck, you unfortunately (or fortunately however you look at it) must do most of the work yourself. Much of the work is really not that difficult if you take your time and learn as much about what you're attempting as you can. This is especially true since you have this forum to ask questions of
Nitrous is always the best bang for the buck with the least work involved. You must however install the bottle inside your car with a Vette (not exactly safe) which also means drilling a hole in the floor for the safety vent. You do have to know how & when to use it or you can make paperweights out of your pistons pretty easily.
Other than that, exhaust will help alot (especially going to headers and true duals). You could also change the heads, intake manifold, and carb without tearing into the engine too far. Just be sure to get a low-rise intake manifold or you may not be able to close your hood when you're done! Using 1.6 ratio rocker arms will give some of the benefits of a larger cam without going through the trouble of actually changing it.
Edelbrock makes a cylinder head, cam, & carb kit that is proven to give 400 hp in a 350 Chevy. These are available through Jegs, Summit, etc. As I said above, you probably can't fit their hi-rise intake under your stock hood though.
Above all, you need to remember this all works together as a system and buying a bunch of mis-matched parts could result in your car not being as fast as it is right now.
My vote for a combo would be:
1) Exhaust (Headers & True Duals)
2) Upgrade Ignition
3) Performance Tune Carb
4) New cam or different ratio rocker arms (new valve covers required)
5) Home port & polish stock heads and intake (or replace with better flowing GM/aftermarket pieces)
6) 3:70 (?) Rear gearset.
7) Open element air cleaner
Stuff like that listed above will get more air and fuel in, more exhaust out, and get the engine in it's powerband quicker. A most basic recipe would be to just maximize the factory set-up as much as possible first.
If you do anything, be sure and save all your stock parts which may be difficult and/or expensive to replace. This is also another advantage of doing the work yourself as these pieces are less likely to "get lost".
Good Luck