So, i'm trying to buy a new computer, most likely from Dell.
But for the life of me, I cannot find how many GHz their computers have.
As a matter of fact, any advertisements i can see for any computers for sale anywhere, they do not specify GHz.

So, is there a way to find out? I do not want to purchase a computer that cannot keep up with modern games...

You aren't going to find it in an advertisement. An ad cannot list everything, just the "grabber" part...what gets you to shop..

Go to the Dell web page. Choose a computer to look at and then look at the Tech Specs that will tell you everything you need to know. Look at all of them, they all give you the specs you need to know and they all pretty much will tell if it is a good computer for whatever.

Look at this Dell page as an example and click on the tech specs tab there it will tell you the various configurations you can choose for that particular computer;
http://www.dell.com/content/products/productdetails.aspx/inspndt_53xs?c=us&cs=19&l=en&s=dhs&~tab=bundlestab (http://www.dell.com/content/products/productdetails.aspx/inspndt_53xs?c=us&cs=19&l=en&s=dhs&%7Etab=bundlestab)

You can have THEM put pretty much anything you want on the computer...it costs more of course than what they are offering as the basic model.

Choosing a computer (http://www.build-a-computer-guide.com/Choosingaprocessor.htm)