I think Google and wikipedia will answer your questions, however I wanted to share my two cents ;
Theoretically hardware differences are minimized between MACs and PCs. However PCs will give you a wider range of choices and mostly will be cheaper whether you build them or buy from likes of HP, Dell etc.
To my surprise in december of 2006, regular price of a MAC pro with two dual core 2.66GHz Xeons was 2500$. My wife wanted one, and I thought I could make something similar for much less. went to newegg.com, checked the processors, $730+ each, checked MOBOs fo dual socket 771s, $300+ cheapest. Checked FB-DIMM, 1GB, $300+
Only these added upto over $2000, plus drives, graphic card, and decent case and PSU would easily add another $300+. A similar system without OS and probably without a decent warranty (except individual component warrantie) was costing well over $2300. I checked, dell, turned out to be over $3000 with lesser graphics card, HP was no better either.
So, last year aaround this time a MAC PRO was the best deal for the given hardware, and we found an even better deal at amazon, for $2200. Of course one could always argue whether that given hardware combination was the best deal. Of course I could not argue with my wife, she earned the money by graphics design work, and she wanted a MAC.
Today one can make a similarly performing system around $700-800 using a quad core 2, sans OS. Same MAC PRO is still listed at $2500, with only an OS upgrade since October.
Long story short, I think there are cases (although rare) a MAC maybe a better deal for the money for a given hardware combo. For 99% of the times though, you'll be saving if you go for a PC.
BTW I'm no fan of MACs, I actually dislike the myth about Aple products andMAC zealots. However, I really admire the mechanics and inner design of the MAC PRO's. I've three Antec , two Alienware cases. They're nice as well, though no where near that sweetly engineered aluminum case of the MAC PRO. I'm hoping to upgrade it to a PC in future, when my wife buys a new MAC pro
OS X is another story, I personally could newer enjoy any MACs just because of the OS, so I newer get why people like the GUI or whatever they like about it. My experience is that it is in general worse than windows xp in terms of stability. Considering limited hardware platform, tight control of Apple, and a big indirect support from open source community, Apple engineers are not doing as good as windoz xp team.