Having thought a bit more, I'd question whether your really need the Prelude. Have you ordered it yet? An X-Fi Xtreme Music, Xtreme Gamer or Titanium would give the same feature set, with bit of a drop in sound quality (remember though - not Xtreme Audio). I'm not sure you'd notice the drop in quality on a PC350 and almost certainly not with the speakers you're thinking of. A Xonar D1 or DX would give sound quality about on par with the Prelude but would sacrifice some EAX compatibility.
The Prelude's a great soundcard but pairing it with the rest of the equipment you've mentioned is a bit like having a ferrari and just using it to go to the shops or the dentist and back.
Edit: Xtreme Music and Xtreme Gamer usually don't have Optical ports but it sounds like you won't be using it anyway. You can buy an adapter if you ever need it.