Unusual behavior
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If the game behaves in unusual ways check your installation above. If you have missing models or begin showing the wrong models for a certain class be sure to check your com_hunkmegs setting from the console. Com_hunkmegs should be a minimum of 96. We recommend 128 or higher.
Rule of the thumb: set it no higher than 3/4 of your available ram, and I've been told bad things happen if it goes above 256, in any case. Although I cannot confirm the latter, I'm running com_hunkmegs 192 on my machine, and I have 512 Mb ram, like yourself. Thus, if I were you, I'd go for
192.
This is how you set it:
com_hunkmegs - The most important command in Generations. Due to the increased memory requirements of Generations, your Quake3 "com_hunkmegs" setting MUST be increased from the Quake3 default of 56. With 128 megs of system memory, players should set their com_hunkmegs to 92. Players with more memory should set com_hunkmegs to 128 or higher, but no more than 3/4ths of your total system RAM.
This command has only become more important in newer releases. As more and more media is created for the game, more and more memory must be allocated to it. Generations Arena loads, on average, over four times the data of vanilla Quake3.
To set com_hunkmegs either set the variable in your q3config.cfg file
-or-
Create a shortcut to Generations Arena with the following arguments:
quake3.exe +set fs_game generations +set com_hunkmegs 128
NOTE: DO NOT run Generations Arena by selecting it through the "MODS" menu of Quake3. This will reset your settings (including com_hunkmegs) to the default values of Quake3. If at any time models begin to defer (like seeing a Doom Chainsaw when you have a Machinegun), your com_hunkmegs are too low.
If that doesn't fix it, let us know.
Quotes taken from the Gen Arena manual, which you should have been reading, instead ;]