Well, what direction are you headed with the car? Daily driver? Street-strip hybrid? Race spec? Show-strip hybrid? Right off the bat, basic mechanical maintenance is both cheap and one of the best areas to start (New oil, oil filter, air filter, spark plugs). Below I listed as much as I can think of that's cheap.

Daily Driver: Port the exhaust, polish up the intake as best you can, get MSD ignition components.

Street-Strip: Same as above, plus remove the spare tire and jack, get a spark amplifier.

Race Spec: Same as above, plus remove the sound system, get a comp cams k-code set (Valvetrain package), consider running a small nitrous system.

Show-Strip: Remove spare tire and jack, port the exhaust, polish the intake as best you can, get MSD components, rims and tires, spring cut.

Ok start off, this is your cheapest place for MSD stuff and other good stuff.

http://captfiero.com/ppc/ - http://captfiero.com/ppc/

Now to performance, Fire451 and I have both built Fiero's that ran as fast as 14.6 with a V6 4spd combo.  My personal best V6 run on motor alone was 15.01 and he edged me out with a 14.99 later that night with cooler air.

  There is a huge and I mean HUGE difference between 15.9 and 14.9 when it comes to performance, however neither of us had any fancy intake or exhaust.  Stock intake, stock Throttle body, only mod was ported exhaust manifolds and no cat converters.

We did have full MSD Ignition, 19# injectors, big cams, custom chips and roller tip rockers.  I ran 1.52 rockers and he had 1.6 rockers.  If you want performance without pulling the engine your first mods are...

MSD Ignition, complete, wires, coil, ignition box and cap and rotor.  Your OEM Cap and Rotor won't handle the 6al box.  Heck if your OEM coil does not have brass contacts it barely works with a good stock coil.

Next mod is adjustable fuel pressure regulator, and fuel pressure gauge. 

Next remove the stock exhaust manifolds and get them fixed.  All of them from the factory are CRAP, you can get 5-7hp just by cleaning up the ports.

Then you start to get a bit more work, but with the biggest gain, 1.6 ration roller tip rockers combined with the exhaust porting is worth 5hp, however the big gain is the increased rpm and power range. 

Oh almost forgot to mention the power pulley set.  You can get it from ASP or I think the Fiero store.  Its worth a documented 1-2 tents in the quarter, thats a good 2 car lengths faster in a street race.  I run the single pulley during the winter and both during the summer.

There are a ton of other little mods like EGR delete, Valve cover breather delete, and remove the mesh under the deck lid vents, all little things that will help the car run better.  Over inflating the front tires to reduce rolling resistance, they can all ad up to an extra tenth.

On a P.S. note, if you are going to race a 2.8 start looking now for a spare motor.  Even with fully rebuilt, balanced and using really good parts, our motors only lasted a year maybe 2. 

If you want to get into more of a full rebuild I can recommend what parts made the biggest difference, however unless you are doing all the labor yourself and your time does not matter, it will cost you more than just doing an engine swap to a 4.9 or 3800S/C.

i honestly will NEVER build up a 60 deg v6 for a fiero again, cost me just as much if not more than it would have cost me to do a engine swap and the gain is very minimal and like dave said i only got a couple of years out of each motor.  if you want to know how to get your car into the 14's the list was like this:

Forged TRW Pistons 9.01:1, Molly rings, Clevite bearings,
ARP Head studs & rod bolts, performance 0.039" head gaskets,
Comp 1.52:1 roller tip rockers, Comp 130 lb valve springs,
custom grind cam .484 ex and .468 int. with 110 seperation
if I remember correctly, Comp Magnum lifters, I cleaned up
the heads, intake and exhaust manifolds, gasket matched everything,
new timing set,MSD Digital 6 ignition box, MSD Blaster
coil, MSD 8.5mm super conductor wires, Heads decked .027" and block head
surface resurfaced (maybe 0.003' off), shaved and realigned lower
intake plenum to match, block was bored 0.040' over, crank was
machined and oil galleries champfered (block as well),corvette fuel pump and mustang 19 lb injectors

also in a stock 87 gt with a msd ignition I was able to get a fastest run of 15.9.