You can use up to 2 alchemy cards in a Mac PCI/PCIX - not in PCI-E
Decklink only supports single card in ANY mac
Is possible to use an Arora+Decklink together... if you can get hold of any arora cards (from the USA) - as it looks like they've gone bust...

Have used all these scenarios above - and they all work in the macs that the cards fit in... You should be able to get an Alchemy for £60 - all others will set you back over £300 min....

hope this helps - give us a shoult if you want to test out a card before you make a purchase

Simon