Hi Shim

Pretty much all VSTi that have come out (Or been upgraded) in the last 5 years have been designed specifically for live use, so no problems there.

A super powerful computer is only needed if you are using it as a complete studio workstation, however when using with a hardware board (Which has plenty of sounds of its own) you will only be using a few sounds to enhance your existing instrument.

The best sounds are expensive, but most entry level VSTi will easily blow away any hardware arranger. (TOTL or not)
NOTE: Where talking sound here NOT styles

The most important things are a high quality external sound card with ASIO drivers, (Assuming it is a Windows system you are using) and a keyboard/arranger that has a comprehensive Midi control system.

This is where your problem will start, as Yamaha Arrangers are awful Midi controllers, (They were never designed for pro use) so there will be a lot of work involved to get it to work to your liking. (Only you can decide if this is worth it)

Hardware modules will be a little easier, but with the Yamahas Arrangers limited Midi capabilities, there will still be a fair amount of work involved.

If you’re still interested, do a search on SZ and the web, (Using variations on VST live as the search term) to get an idea of what you want.

Here are some links to help you get started

http://www.tweakheadz.com/how_to_get_started_with_midi.html
http://www.tweakheadz.com/audio_interfaces_1.htm
http://www.synthzone.com/
http://www.kvraudio.com/

Hope this helps

Bill