I know there's a few "generations" of that drive that have very minimal changes (some don't have FireWire, others do, etc) between the Live! and X-Fi PCI cards - my general advice would just be to look up whichever card you have (or are going to have), and find out what the "Platinum" or "eX" or what-have-you model came with as far as a drive, and get that drive. If I remember right you actually can mix and match between generations to an extent, but I wouldn't personally attempt it if you're going with a card that shipped with bays that lack 1394, and you're hooking up a bay that has 1394, as there may be some pin incompatibility.

You will need to ensure you get a card with AD_EXT or AUD_EXT (depending on which generation of board you get), to have the proper connector for the drive. Many of the value/discount/etc cards lack these connectors, so you'll usually need to buy the "Vanilla" variant - for example the Audigy 2 ZS can plug into the Platinum's front bay, but the Audigy SE cannot.

As far as using a 40-pin IDE cable - it does work as long as you get one that has ALL pin positions open, and can line it up properly on both ends. Alternately just look for an "AUD_EXT" or "AD_EXT" ribbon cable, if you want something that's properly keyed (it can make installation easier). Shouldn't be terribly expensive.

Also note that the "Pro" or "eX" external boxes will only work with that card - those boards have external connectors that hook up through a proprietary cable (e.g. the X-Fi Platinum Pro or Audigy Platinum eX).