If your Realtek AC97 is one of the new High Definition chipsets then it probably won't bring you that much difference unless you intend on playing games which fully utilize EAX 5 on the X-Fi card - The titles you mentioned, bar Quake 4 can utilize EAX 2 which is handled by the Realtek AC97 chipsets anyway.
If your Realtek AC97 gives too much noise, you play lots of EAX 5 compatible titles, and you intend on upgrading your speaker system to 5.1 or a very good set of headphones then go for the X-Fi. Otherwise if you have a set of £10 speakers that you will be stuck with for quite a while, they will be wasted on the X-Fi despite its awesome technology. Just like buying an expensive High End, £1k, 110W per channel, THX compliant Home Cinema amp and then powering some cheap 50W Eltax Speakers from Department store catalogue - it will work and sound better than a cheap amp, but the abilities of the expensive amp will be VERY wasted.
However, if you are so hell bent on getting yourself the X-Fi, set aside some money for some good Creative 5.1 speakers, preferably the Inspire, I-Trigue or even Megaworks Series if you can afford it, statying clear of cheap sector of their product range. By all means spend more and go for high end companies or even full home theater receivers to put into this soundcard, but if money is tight then the Creative Inspire is a good entry point into the world of Surround Sound :lol: