Trial & Error. These questions will all depend on the type of process/scripts that you're running.
Will a forum with 1,000 visitors a day operate on a P4 2.4Ghz with 512MB ram? Yes it will. Will it run better with more memory? Definitely. Why? Because forum scripts that depend on MySQL or InnoDB are memory intensive programs. The more memory you have, the faster it can spit out information. If you have a lot of people searching on the server then you'll want more CPU to handle the requests.
Just like a personal computer - will you be able to run Windows on a P4 2.4Ghz? Yes you will. Will you be able to have 10 windows open browsing the web? Probably. Will you be able to load up Outlook, 2 excel sheets, 4 Word Documents, 8 Browser windows and a music player and except everything to be running smooth? Probably not.
It all comes back to Trial and Error. Someone somewhere took the time to test out World of Warcraft on a computer and make the determination that you need "X" memory as a minimum requirement, and "X" as a recommendation.
So if you know what scripts you're wanting to run, and what your final task is that you're looking to accomplish, maybe we can help guide you as to the hardware that you need. There's plenty of people on the web that have likely run into the same questions and provided answers.
So, real life example for me;
10 Firefox windows open (each with 10-14 tabs open)
8 Internet Explorer windows open (each with 4-6 tabs)
Outlook for mail
Digsby (MSN, ICQ, AOL, Yahoo chat) - 5 conversations
windows media player
7 notepad windows open
16 excel sheets
8 shell scripts (putty)
Dreamweaver
Fireworks
LiveZilla (live chat for my website)
Nagios (server monitoring program)
Adobe Writer (finishing up a contract)
Skype (chatting to a co-worker via Video & Voice)
Quicken
I have 4 x 24" monitors, 8GB memory, AMD 9500 Quad Core 2.2ghz Processor and running Windwos Vista 64-bit. I get ZERO lag when changing between programs.
Am I overkilling the resources? Nope. Did I buy outside of what I really needed? I don't think so - I may have went a little heavy on Memory and could have used only 4GB to accomplish the same tasks.
But this is what I do DAILY. Just so happens, this is my CURRENT desktop setup at this moment. Once I"m done in these forums, I close out about 14 tabs of FireFox, and I'm off to the next set of forums or work I need to do.
Oh, and since I like to multi-task, I do have 2 notebooks and another computer beside me with 8 monitors used for monitoring server networks etc mainly.
So it comes down to what YOU want to do, and how you want to do it. I can tell you that my truck is great and you should buy one (Dodge Durrango), but if you need to haul an RV that's not the vehicle you need. So what do you want to do, and what are you trying to accomplish?