318 combos?

There are a TON of threads on this. Look in the SB subsection of the engine forum for some, or do a search.

300-350hp is pretty easy to accomplish with a rebuilt 318. The basic formula looks like this...

Bore and zero deck the block

Select pistons to get you around 9.5 or 10:1 on the compression ratio (kb167's are a popular choice for a street engine)

For 300 hp you can have your stock 318 heads fitted with some 1.88/1.60 valves, and have them port matched to the intake. A little clean up on the ports would be good, but I wouldn't waste money having them ported (see below)

For 350hp or more, look at a set of magnum heads, or the new rhs x heads (400+ hp)

Edelbrock performer rpm or rpm air gap for an intake

Lunati 60402 or 60403 or similar on the cam, depending on your brand preference.

Dual exhaust if you don't already have it, manifolds are fine for 300 hp, headers if you're going bigger.

Here are a couple magazine articles on 400 hp 318 builds...
http://www.moparmusclemagazine.com/techarticles/mopp_0409_318_engine_build/machining_process.html

http://www.hotrod.com/howto/113_0304_318_small_block_build/index.html

And seriously, do a search. Good luck!