Thats AMD for you. They run at slower clock speeds than Pentium processors but they can do more a clock, making them faster. Basically if AMD specifies at 2.2ghz, and is listed as 3500+, its as fast or faster than a 3.5ghz Pentium 4 in some tasks.
AMD are better gamers than Intel Pentiums, but Pentiums have the edge in video editing/encoding and the processor intensive type tasks: even more so on some programs if you have an Extreme Edition (Basically a true CPU core, and a theoretical CPU (unworked clocks go to do something useful, sort of emulating dual core performance)
Though, yeah, right as of now, if you have the money, and since prices are dropping, I'd seriously consider an X2
As for Dual Core, if its specced at 2.8ghz for example, thats both chips.
With Dual Core though, its alot better, and if programs work with Dual Core (more and more will), you'll get alot more performance.
Still though, alot of programs dont work with Dual Core, so you won't get a performance boost over a single core processor.
That said though, Windows can work in Dual Core mode, so for instance, all the main processor tasks could go to one processor, and then, your music player, Internet, for example could be running on another, speeding tasks up.
Another good example would be one CPU handling a game (a game running in single core mode, but windows running in Dual Core mode), and the other CPU handling an Antivirus scan at the same time! (Something Single Core CPU's find VERY hard, if not impossible to do).
So, yeah, some things worth thiking about
Thats full dual core useage right there, and when prices drop, it'll be great value for money.