Welcome to the forum.
Where are you at? it'll help find nearby shops and prices.
Overclocking is speeding up the cpu by fiddling with options in the BIOS.. It makes more heat though.
You just need a decent video card, power supply, case, dvd drive, and hard drive. Oh and Windows
If you really want to be cheap, and have no pets, you don't need a case
And yes any old dvd drive will work., and if your freind has an old copy of of XP thats just fine too.
4Gigs of RAM is just fine.
According to this:
Maximum Detail Benchmarks : Diablo III Performance, Benchmarked
This setup for $128.00, Will get you 55fps
Newegg.com - Computer Parts, PC Components, Laptop Computers, LED LCD TV, Digital Cameras and more!
The rest is more personal..