Shoud I upgrade from Audigy SE to X-Fi extreme music?

You wouldn't notice a difference with those speakers, PC speakers do not really benefit from expensive sound cards.

X-Fi would not really sound better on your system, unless you used more expensive speakers or headphones.
I own an X-Fi and the reasons i bought it were:
1) Virtual speakers over headphones (it really works great!)
2) 24-bit Crystalizer (although it is not a sophisticated solution, it just tunes the sound frequency bandwidths, it works surprisingly well, making all those MP3's sound at least twice as better!)

So if you are an audio fan, then the Crystalizer should change your life to the better for sure, if you are just a casual gamer with the same setup, then the only thing you get with X-Fi is support for EAX 5.0. The quality will only get better by a small margin.

It is true about the software though. It is still very bloated, many useless features and some bugs and annoyances. I am using WinXP and it runs ok there with Direct Sound acceleration, but if you are using Vista forget it. You will need Alchemy (Creative's solution to accelerate OpenAL, by translating Direct Sound calls from games to OpenAL).

For a more all round solution i would go for a PCI-e ASUS sound card. They have better specs, much better drivers and they even emulate EAX 5.0 calls (not 100%, but still very close). The price is a bit high still, but they should go down in time, especially if another 3rd party jumps in the market.