Do you mean the z5300? Because I've never heard of Logitech z5000 speakers.
So you would like a sound card with support of 5.1 speakers I suppose...
If you want great sound but dont care about extra features you could opt for the Creative Labs SoundBlaster Live 24-Bit. On TigerDirect.com it sells for $33.99 (US). You could also buy from other brands like AOpen or Chaintech to spend even less. Creative is the best for sound though.
I checked on misco.co.uk (same as TigerDirect but in UK) and the Creative Labs SoundBlaster Live 24-Bit is £22.99.
You could also get the CREATIVE SOUND BLASTER AUDIGY2 ZS 7.1 THX for £54.99.
EDIT: I just saw your other post. The speakers are the z5500!! Nice. I have z5300e and they are enough for me in my little room :D. I had the z560 but the subwoofer and its connectors got screwed up so the sound wasnt there or distorted. Luck for me it was still under waranty. They sent me the z5300e with no charge and didnt have to send back my other system :).