I'd like to let you know if you didn't already the X-Fi Xtreme Gamer does not have optical digital out, it doesn't even support the digital I/O module. Secondly, the X-Fi Xtreme Music also does not have an optical digital out, you'll need the digital I/O module (£30) for SPDIF ports.
To connect via digital (optical), spending over £70 for a soundcard is madness. The only two (or three) reasons why anyone would need a soundcard in this situation are for games or surround sound.
The DAC circuit in a soundcard which determines analogue audio quality is not used when outputting via digital, it can be the difference between a £130 card to one costing as little as £10. A good example of this are the C-media based cards, some can be bought for £10 where others like the ASUS Xonar go for £100+, via optical (two channels) they both sound exactly the same but through headphones or a direct speaker connection (non digital) they're worlds apart.
If gaming and surround sound isn't a key factor any cheap £20 soundcard which features optical SPDIF out will do, providing there isn't already an optical out on the motherboard, in which case you're already set.
If environmental effects in games is something you're after you need a card which supports EAX5 for example, the X-Fi series do this very well (except the xtreme audio), the ASUS Xonar cards are also something to look out for in this field. I personally recommend the X-Fi Titanium for this.
If surround sound is important you'll need a card which can encode multichannel audio, the X-Fi Titanium is a good choice as it encodes DTS and THX plus features optical in and out ports.
To summarise, if you're a gamer and/or want surround sound go for the
X-Fi Titanium. Note this is PCI-E x1, if you lack PCI-E slots I'll find you an alternative. If games and surround are not important and you just need a digital SPDIF port then any cheap card will do e.g.
Terratec Aureon 5.1.