Galvanized has some good points here. You'll have to evaluate the pros and cons on the basis of your preferences for time, money and bargains.
Maximum PC Magazine sometime in the last year or two had an extensive article on "case-painting." As I recall -- and as I myself would have recommended anyway -- the process pretty much follows the imperatives of automotive-body painting.
You should sand, then primer-coat, and spray-paint the enamel. For automotive work, one learns quickly that the spray-cans you buy at the auto-parts store just don't do the trick -- you're better off getting cans of DuPont automotive enamel, reducer and other incredients and using a paint-sprayer.
But for a computer case, it may be possible that spray cans can do the trick. If the work turns out "looking" like it was done with spray-cans -- with grainy marks that show a pattern like tagger-graffiti of LA street-gangs, you could fine-sand the resulting work with water and 400-wet-or-dry, then clear-coat with polyurethane. Even without the clear-coat, you can often buff out the irregularities with 400-wet-or-dry and then polishing compound.
But for the time spent -- it should be a labor of love that doesn't recognize economic scarcity. A paradox, since you're choices do -- in fact -- pay respect to that very principle.
Maybe you should experiment with some metal panels before you commit paint to a brand-new case.
My feeling is similar to Galvanized's -- if you're going to spring for a bargain-sale on a $160 case for $115, you might just as well front the extra $45 and get the color you want.
Now if you were resurrecting an old IBM-beige ATX wreck from the 1990's (see my posts -- Galvanized knows . . . ) it's a different matter . . .
