I'm wanna record guitars, bass & vocals with PC. I got Cubase 5, Guitar Rig 5, Nuendo & Amplitube. What softwere can you recommand me for vocal recordings? Can it sound decent for band's demo?
Your machine and interface/converters are what matters here. The DAW is the piece in the chain least likely to impart anything to the signal. It's just a tool. It's what you do with that tool that matters.

I LOVE Reaper. Check it out. What kind of interface do you use?
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For vocals I usually use Terry West EasyVox, Epicverb, TAL-Chorus and ADT Vst (For some doubling effect). All free. Except for Antares Tube that I got for some dirt.

Those are what I use. Others might recommend I other stuffs. But remember, plugins don't give you "decent" right away. You need to LEARN your plugins and learn how to mix properly.
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You shouldn't really need much software for vocals if you have a good microphone and have a good singer. http://www.kvraudio.com/ < this site has tonnes of free and commercial vst plug ins you should check out, it's really best to check a few of them out and see what suits you best!
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Have you considered the alternatives before committing to recording with your PC? Get yourself a decent multitracker & you won't need anything extra to record vocals, they contain everything you need in a single package.

If you're wanting to use it for your band's demo though, you should rent a professional studio for the day and get it done properly. Recording is a skill like any other, it takes time & practice to master it. Your first attempts won't be good enough to use as a demo CD.
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