Windows XP is actually more efficient (programs takes less machine resources) than later windows versions. It is the quality of the hardware that counts. And chip performance keeps going up every few months while the price goes down (google "CPU benchmark 2014" and "GPU benchmark 2014" before buying a PC).
I recently bought a new PC with an Intel i7 4790K 4.0 GigaHerz quadcore processor which costed only 289 euro (total price of PC, including 16 gig memory and fast 250 gig SSD diskdrive, was 1480 euro). Caveman2cosmos runs excellent on it.
I decided not to wait for windows 10 as new windows versions often have bugs. I may switch to it but definitely not before service pack 1.
Windows 8.1 has some annoying features compared to earlier versions of Windows, but it still works and 90% of the time I'm inside a program anyway. I heard that installing a free program called "classic shell" removes all the bad parts from Windows 8.1 .