Also also a CPU running natively at 5Mhz would explode with 128Mb ram, most PCs back then didn't even have a full 1Mb.
I have to nit-pick this, for the benefit of anyone who's curious...
The original 8086 used 20-bit addressing, which means memory was limited to 2^20 bytes = 1,048,576 bytes = 1 MiB. It had no support for "virtual memory", like we have today. So, software could only address 1 MB and the CPU package surely had only enough pins exposed to address that much.
I take it your point about it "exploding" was a reference to the fact that a PC case would overflow with all the DRAM chips it would've taken to reach 128 MB? Back then, RAM wasn't on DIMMs or SIMs, but instead you actually plugged the chips straight into the motherboard.
There's an interesting footnote, which is how 20-bit addressing was implemented in a 16-bit CPU. The way you'd compute an address is by adding a 16-bit "segment" register with an offset. This created an interesting technicality, because 2^16 * 16 + 2^16 = 1,114,112. So, when the 80286 came along and added a different addressing mode that meant it could address up to 16 MB, someone thought up a clever hack of using that top 64 kB from "real mode". This address range became refered to as "High Memory". If anyone remembers fiddling around with loading
himem.sys, for certain DOS games, that's what it was - just a way of squeezing out a little more memory to be accessible from DOS mode, assuming your machine had > 1 MB of physical RAM.