Getting Started With Virtual Instruments, Mac as a sound module: recommendations? Options

[mason]

I've used MIDI sequences with a live band for many years. Generally my set-up has included an old mac laptop running Metro (Sagan Technologies) plus hardware tone modules.

Question: What's involved in using a Mac as a sound module (instruments). How powerful does the Mac need to be (speed, storage, processor)? In terms of instruments, my requirements are simply: bass, drums, keys, horns, strings, organs, pads, etc. Quick loading of songs is a must.

Any recommendations on compatible sequencing software and "instruments?"

If a little older technology will do, I'd like to avoid buying a new Mac.

Thanks,
Mason