Forget the Reaper internal figures for a moment, they do not tell you about the nasty peaks that require more beefy CPU and generous buffer settings.
General recommendations for those will gutless old hardware:
Do you have any known CPU hungry VSTs?
That's all you need to ask yourself really. If the software house recommends a more powerful CPU to run their VSTs. Consider how a lot of instances will soon gobble up your CPU.
If your CPU is not up to the job make concessions regarding the number of voices in polyphony and sound quality settings within the VSTs.
Also limit the projects to 44.1khz.
Run the most hungry soft synths (if you have any of the usual suspects) in mono or duo mode rather than poly modes. Consider a mix of weedy older soft synths or freeware and 16bit sample instruments instead of more CPU hungry soft synths.
Also you may have to consider freezing tracks. Oh, and don't be too ambitious with latency settings.
In general though remember less is more - because you won't really have a choice!