If you want to save money download Reaper for free. Its an easy to use audio recording program that handles the recording and sequencing duties. I would also suggest, to connect the PSR-540 keyboard MIDI output to the MIDI input port on your Emu 0404 interface via a MIDI cable. This way you can use the keyboard to trigger internal VSTi's and samples which will give you alot more options.

As far as not getting enough signal I would ask how did you connect all this up? The mixer can be connected to the analog in of the Emu and be used used to bring in mic and inst. If you connect the mixer input to the Emu output it can be used to monitor your mixes via headphones or studio monitors.

The Vamp has a digital out that can be connected to the digital Spidif inputs on the EMU.

I would suggest reviewing this link it might help to sort things out
http://www.tweakheadz.com/guide.htm

There also alot of free VST's and VSTi's at this site to get you started
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I think the equipment you have is enough you just need to set it up properly and adjust the levels properly. Post back the way you have everything connected now and people here are really cool and willing to help you get everything hooked up right.

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This may be the reason for your level problems.
Just about any software package will do the job. Most software companies post demos on their web sites. The only way you can tell which one will be best for you is to try the demos and see which one you like the best. What I like you may hate and what you like I may hate.

I used acid pro 6 for a while, It's a decent audio handling loop based program. I recorded everything in it and managed to get some decent mixes so...
Sound forge isnt for mixing its more for editing audio and wave form blah blah... vegas is more for video isnt it?