it's really a matter of how fast that horsepower is applied, not necessarily the total amount.
you can have all the horses/torque you want, but your tranny can only spin so fast before it starts redlining (i have a manual gts and redline is 7800rpms)...and obviously driving in the redline for a relatively long time (over 5 seconds) can damage your tranny if you do it constantly enough.
as a rule of thumb, you should be able to add 100 more whp (wheel horsepower) gradually without needing a new tranny--but you still will only be able to go about 170 mph before you hit redline. (thats what it is in a 6spd anyway...4 spd i don't know how big the 4th gear is).
adding horsepower/torque really isnt meant to add top-end speed, it's meant to accelerate faster. thats the limitation of your tranny.
anyway, adding bolt-ons will get you NOWHERE near 100 whp...you can add exhaust, Intake
, header, Turbo, nos...whatever..and your still only looking at about 50-60 whp gain...all of which your stock tranny can handle easily (although replacing your clutch with a more aggressive one is recommeneded--the stock clutch is rather weak. if you plan on adding a significant amoutn of horsepower/torque...a new clutch/flywheel should be at the top of your list