i dont think you would need to add any vent holes to the plastic case. there's an air-gap in the bottom where it meets the metal frame which allows it to vent, probably more than sufficient for your needs. plus, why ugly up the case?
also have you considered widening the inlet holes toward the front of the case? Especially if you're increasing the air output, there's a potential that there may not be enough ventilation to pull in cool air in which case the effort of the new exhaust is wasted.
you might, if you're feeling daring, want to replace the fan in the PSU. ive found those are usually the worst culprits. but, if you're buying a new PSU, it may not be an issue.