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.