It depends how serious you are about things. You can spend literall nothing...ie the digital fish plugins and do a fine job, or you can spedn 1000s of doollars for ones that emulate their esoteric hardware brethren.
Free ones worh getting are
Digital Fish... a whole sweet including compression etc
Buzz audio... useful little collection of various freebies
SIR convolution reverb.. free and excellent .. you can find impules to load into it from quite a few sources
PSP quite cheap considering the overall quality is actually very very good
Sonalkiss do a nice sweet of plugins as well
Waves very expensive and , frankly. slipping behind the game
UAD, a bespoke card that takes the strain off your own CPU, even the starter pack has some excellent software recreations of things such as the 1176, the add ons you can buy are some of the best software sims available.