Just some remark for the power required for a PC: yes, a Pentium can fold technically but it will take quite some time to finish some of the work units, could be easy more than a day.

It is expected that once a user start with a work unit (WU) it should be finished for the simple reason as the WU are sequential in nature. When such a sequence get interrupted by not finishing in certain timelines the overall science progress get delayed.

Plus modern GPU will get much more points will much less energy. So if your goal is folding & money: don't go in with too slow equipment.

Is your goal is primary the science: every hardware is useful and welcome.

There is also a client running under Google Chrome browser (NaCL). That one has smaller WU which easier could be finished on a Pentium.

That said: a Pentium driving two or three GPU is a great use of older hardware.

Welcome to folding ...