Reliable office PC recommendations?

Yeah, honestly, people don't realize that Windows isn't always smart enough to tell you the full story. If you've got an Intel chip with hyper-threading, Device Manager might show you two cores when it’s actually just a single physical core—it can be pretty misleading, right? It's usually best to just head over to the Intel website and check under the specific core category to see how many actual, physical cores you're really working with.

Right now, I’m running this old piece of junk at 2.8 GHz with 2GB of RAM and a 40GB hard drive. I need an upgrade that actually feels fast, but some things just aren't adding up.
I've been looking at http://www.ebay.com/itm/Dell-Desktop-Specs... which features a Celeron at 2.58 GHz. I'm confused—how can the clock speed be lower than my current setup? My current chip is ancient; I bought it used years ago and honestly don't even know what it is. Am I wrong to assume this Dell would be faster despite the lower clock speed?
My typical workflow involves running a POS system, Microsoft Office, Winamp, a browser, and occasionally an image viewer. It slows down, sure, but it gets the job done. I'm running Windows 7.
I figure buying the one mentioned above would give me a nice boost thanks to the extra RAM, but that clock speed thing is still bugging me. Even after poking around on PCPartPicker, it looks like the newer tech should be twice as fast.
Am I missing something here, or should I just pull the trigger?

What processor are you running? A Celeron D on a 775 socket?

Clock speed probably isn't the best way to measure raw power... I mean, that only really works if you're comparing chips from the same architecture, and your Celeron is likely six or seven generations old by now.

You could definitely get a better build for that kind of money... maybe try a micro ATX case or even a slim mini-ITX setup?

The main thing is I need to be able to pay via PayPal and it has to come with a licensed version of Windows. Everything else is secondary—I just want something cheap that runs faster than what I've got right now.

I guess that’s really your only move there from what I can see... though it will probably be faster.