I can tell you that when it comes to bang for the buck, the difference in backbox speakers is very minimal if you also do the l-pad that Mark mentions. I decided a while ago to upgrade my Viper Night Drivin's sound, and once I got the sub done never bothered with the cabinet speakers because it made the whole thing sound five million times better.

It's basically the same process for any game, it just depends on the game that you're doing. Mousin' Around is an OUTSTANDING game to upgrade, as I assume Space Station would be (hmmm... I should try that one!) but others don't have the low range that makes it as worth it in my opinion.