I would love to see a recommended system requirements for ..... in one central location .... 
the classics .... dk and pac ect ...
Pretty much anything in the last 5-7 years should run these fine,  including most cell phones most likely.
the high resource games ... street fighter 4 , blitz and others 
4ghz+
new shooters , daphine games  ... 
About the same,  and daphne runs fine on anything in the last 5 years easy.
I would love to know .... that uses what .... roms use RAM ?
Yes,  they're loaded into RAM during gameplay.  CHD's are streamed into memory IIRC.
How much ram do I really need ?
About as much as the largest ROM you're going to load + OS req's.
My OS also has limits ?
WinXP and older is limited to 4gigs,  Vista 64 and Win7 adress tera's.
Others are CPU driven ?
All of them are at present,  this may change in the foreseeable future,  Mamedev has been rumbling about some GPU offloading.
Dont I really want a single core processor ?
No.  At least dual core.  First,  Mame can use multiple cores,  thought atm it's optimized for 2 cores.  Further,  your OS is still running in the background,  so it's eating some CPU cycles.  Ideally a quad-core would be best with Mame set to use 2 cores,  leaving the OS two cores for it's work is ideal for multi-core performance.  You'll see a benefit,  but it's not going to make Blitz playable on a 2.8ghz.  Just give you some added percentages.
Video Card ?
Now,  they vid card doesn't matter.  It's possible this may change in the foreseeable future.
Where does my Hard Drive coming into play ? Can I put all my roms or mp3s on a 5400 rpm HD ?
The drive is used when the ROM is loaded,  and that's all.  So if you don't mind waiting a few extra seconds,  5400 is fine.  Though with 7200rpm drives costing less than a video game,  I can't imagine why you'd want to buy one.  CHD's may perform a little better with 7200rpm drives,  as they're streamed (more or less).
Any my OS and frontends on a SSD drive ?
Keepign your OS on one drive,  and the stuff you're running on another is always a good idea.  An SSD has benefits,  but not for their price right now.  With terra's under $100usd,  there's no reason to buy an SSD  for hundreds.
I'm kinda getting the impression you're trying to use Mame fon a laptop.  This will never be optimal.  Laptops have all kinds of performance reducing issues,  they're heat-limited,  dependent on power cords for full speed,  and slower than a desktop.  Any performance ratings for a desktop is going to be lower than what you'd need with a laptop.