if you're stacking crit, you may as well stack surge as your crits won't do too much without it. i think people tend to get 35-40% crit and at least 70% surge. of course it all depends on whether your skills benefit off crit more than power or visa versa. as a sharpshooter gunslinger, i know that crit is your best friend. not really sure about the other trees though.
alacrity is mainly for healers. you shouldn't even be messing with aim as that's the trooper's main stat.
The typical "sweet spot" for surge is at around 75%, that is exactly five pieces of end-game gear. The optimal crit value seems to be around 35% w/o stim, maybe a bit more for sharpshooters. As far as augments go, you want cunning in most slots, maybe some power as well if your crit % is high enough already.
As to aim, you shouldn't ever gear for it, but nothing says you can't get the appropriate datacrons, after all they do add to your ranged damage and crit chance, and as a dps you want to squeeze out as much hurt as possible.