Eh, I'd say that obviously a good quad is better than a same-class CPU with two less cores, but it's not so simple.
Compare an i3-2100 to an Athlon II X4 630 for example. The i3 I believe is faster in everything, even encoding thanks to HT and much much better IPC.
For a general light-use (typical non-gamer/encoder/pro usage) web/email/facebook/angry birds/word/excel kind of user, there's nothing wrong with a decent budget dual core to be honest. As a matter of fact, looking at Windows 7 and typical budget hardware these days, it's never been so good.
Think back 10 years when Windows XP was new. A "cheap" PC was $800ish, and had only 128 or 256MB of ram, a slow ass 5400RPM (in some cases 4200RPM!) hard drive, terrible integrated video most often, a weak CPU, and so on, making even basic computer use kind of painful. Even more so with a crappy antivirus program bogging things down even more.
Today a $300 PC from WalMart with an Intel Dual Core / 2GB or 4GB of ram / 500GB or larger HDD, etc, will run basic tasks extremely well.