Q) If a card is discarded, does this also prevent the effect of any action it has performed earlier in the turn?
In this case Discard is similar to disable. Disabling a card prevents it's card text from being used, just as discarding it would. Card text for an action is used at the moment the action is performed, not afterwards. For Example, All Forward Disruptor Banks reads: "Action: Disable this card to add +1 attack die to an attack in your forward arc this round."
The action must be performed after your move and the action disables the card. Any attack would occur after the card is disabled, so the effect of the action must persist beyond the disabling or discarding of the card.
Q) Can you discard a disabled upgrade to fulfill the cost of a card that requires you to discard an upgrade?
A) In general, yes. The rulebook describes the effect of disabling a card
as "a player cannot use it's card text". It is still an upgrade and
still (for example) a crew card as this is not part of the card text.
https://dl.dropboxusercontent.com/u/30223400/Official.jpg CGuild: http://boardgamegeek.com/article/16177280#16177280 (thumbed)