If this thread could be bottled, we could fertilize the sahara.
It's handy for multitaskin (and as mentioned, rendering, ripping etc), that's all. Windows is hopeless at managing multiple processes, so having an idle CPU to drop new processes on is, a benefit, to it.
(my beloved linux is normally much better, but when it gets it wrong, it gets it WRONG).
An example of quad improvements would be seen in the way your system runs while unpacking a large rar and some other hefty task.
It's NOT anything to get excited about though, and totally totally pointless if flat out speed is your goal (as has been said, a dual will do better in this.....and I wish I had one).
BTW, the "quicker login" thing, is quite simply the only scenario where 4 cores is a massive help over 2.....because during that time there are DOZENS of processes being launched, reg mods being made and repeatedly read, it's the busiest time of the computer's working day. Not in the least representative of what happens once it's finished logging in.
Quads are neither the proverbial canine gonads, nor a complete bottle of snake oil, but, as a quad owner, I'd say their more snake oil than not, until we see some software that threads well, or an OS/mobo arch' which can distribute load among cores.
My dual box is zingier than my quad, but that's cos AMD are zingier than intel, single and dual intels are just the same......hard, if not impossible to define....it's like there's a moment where the machine is distracted or something when you go to launch stuff, I really cannot put it a better way. Only seems to be launching stuff that it happens with, once stuff's happening there's no problem at all.