The Ubuntu family, the most commonly used:
|CPU Req||1 GHz Celeron (better recommended)||1 GHz Celeron (better recommended)||1 GHz x86 (better recommended)||Pentium III-750 (Core 2 Duo 1600 MHz or betterrecommended)||Pentium III, Athlon XP (better recommended)||Pentium II, Athlon (better recommended)|
|RAM Req||1 GB (2 GB or more recommended)||1.5 GB (more recommended)||512 MB (1 GB or more recommended)||512 MB (2 GB or more recommended)||512 MB or more(minimum install can be done with 128 MB)||128-384 MB (512 MB or more recommended)|
|OS size||7-16 GB||7-16 GB||4-10 GB||8-16 GB||2-6 GB||384-800 MB|
Most systems nowadays can take at least 8 GB of RAM and a speed of 1.5 GHz for a CPU, APU or SOC seems the bare minimum now. You may have some older hardware lying around though.