Well, if you have a old Voodoo5500 laying around you can do this and the game will run excellent,I get 75 fps steady in the game and no glitches.
First if your on XP you need the right driver for xp get it here. 
http://www.voodoofiles.com/12447 
You want the AmigaMerlin3.0 XP Driver.
Unzip to any folder and install it.
XP will say not to run the driver but install it anyway. The driver will load and run fine. 
Now go to the Postal2.ini file in the postal2 systems folder,
(If you dont see Postal2.ini run the game one time. If you crash on the v5 it will make the file)
Double clik it, or open with note pad. 
Set the settings as below:
( Make sure you clik File at the top of the browser and save after you make the ajustments).
 
[Engine.GameEngine] 
CacheSizeMegs=64 
 
 
[D3DDrv.D3DRenderDevice] 
UseTripleBuffering=True 
UseHardwareTL=False 
UseStencil=False 
UseVSync=False 
LevelOfAnisotropy=0 
 
Just a little Voodoo help from TheOutlawDad for all the die hard voodoo guys,
(Macoo) who told me the game would not run on Voodoo. lol,  
 
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TheOutlawDad http://postal2.tk