You could probably find yourself a pretty good 2nd hand Pioneer AV receiver in your local catalogue overflow shop or whatever, or on the bay.
You mention that you've just bought an X-Fi, did you get the Xtreme Music, or one of the premium models with a bay? The Xtreme Music does not have an optical output as standard, so you'll be stuck for optical there.
Since it sounds like your only looking for a stereo receiver, analog from the X-Fi could perhaps be fine with your current amp. Wait for your X-Fi to arrive, and see how it sounds with what you have now, before going down the road of upgrading your amp and/or speakers.
The AV receiver market is complex, with many different makes and models to choose from, and the same goes for speakers. Prices range from £50 to over a grand just for the receiver...