Hi RobP,
First of all, it's sad to know about the loss of your Yamaha SW1000XG sound card. I think it was one of the best 16bit/48KHz sound cards out there, with a very powerful sound chip, although I never owned one (I got the cheaper Guillemot Maxi Sound Fortissimo with the Yamaha YMF744 legacy sound chip).
Now, regarding your questions, let me say first that using a sample library (whatever it may be) requires a more complex setup than using SW1000XG onboard XG bank. Let me elaborate this. The SW1000XG has a dedicated onboard XG bank, which contains the sounds you hear while scoring in Personal Composer. That bank is embedded in the sound card's hardware and is recognised by Windows through the sound card's drivers. This allows any audio software, like Personal Composer, Cubase, Sibelius, or Finale, to recognise the XG bank as a valid MIDI device, and use its sounds directly.
The same doesn't happen with most sample libraries. Windows can't recognise them as audio/MIDI devices, because they aren't that. So, no audio software can access them directly as stand-alone programs (how I wish they could...

). Instead, sample libraries run inside other audio software, as plugins. In Windows, VST is the most common plugin format. GPO comes as stand-alone software and VST plugin.
Now comes the tricky part for you. Sequencing software, like Cubase and Sonar, can host VST plugins since long ago. Notation software, like Sibelius and Finale only recently started hosting VST plugins, to compete with sequencing software. But Personal Composer (PC) doesn't have that (valuable) feature, for what I read in their website...So you can't use GPO as plugin in PC.
Nonetheless, it is possible to use GPO, as a stand-alone software, with PC, as long as you direct MIDI data from PC to GPO. This can be done by using a virtual midi cable software, like MIDI Yoke. This software emulates a set of MIDI channels with corresponding inputs and outputs. PC would link to MIDI Yoke's outputs (visible within PC's MIDI playback configuration, or something alike) and GPO would link to MIDI Yoke's inputs (visible within GPO's audio configuration), thus routing MIDI data from PC (virtual cable output) to GPO (virtual cable input). The downside is that you would have to manually set up each individual MIDI channel from a PC staff to the corresponding GPO instrument, in the ARIA Player (GPO's new sample player, since version 4.0).
Another solution would be to buy GPO4 and another notation software, like Overture (which can host GPO as VST plugin), or Notion SLE - GPO Version (a reduced version of Notion3, that only hosts GPO, but is cheaper), from Notion Music. From what I've read, Notion SLE - GPO Version has an embedded soundset for GPO, allowing the user to focus only on scoring, rather than having to set up the audio configuration every time a new score is started. You can read more about this in their website, if you want.
Best regards.
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