rim sizes

do you mean from a legal point of view or just, whats the biggest that will physically fit?
legally, it depends what state your in and what bull**** laws say you can do.
if you where putting 20's on your VX id say to look at getting coilovers for the front, you'd get away without having them but im pretty sure your suspension travel wouldnt be legal or for that matter perform very well.
alot of people i know with 20"+ rims on VT-onwards model commodores with fairly standard sort of suspension or lowered a bit are usually sitting on their bump stops.
not sure about standard height, but i know for a lower commodore with big rims, rolling and even pumping the guards is usually needed.

 

go 20's man, they look killa or if ur not to keen on 20's at least go 19's,

 

there wont be too much comfort with 20's on it, lol.
i think 20's are a bit dearer than that - especially after putting them in half decent rubber, the more you skip on that the more you save. i would think $3k would be closer to what you'd be up for.

 

personlay, id steer away from 20''s on a vt-vx. one ov my mates got some and to avoid exesive scraping he's gotta put 225's on em without rolling the gaurds. witch looks dumb cooz the rim sticks out to far. i put 19''s on mine with 245's and i got no probs at all. they set me back $3000 btw