I became concerned if we were still in Pentiumland, so I asked him, "when you say 'D,' do you mean 'Pentium D?' I don't want to change that, but otherwise, I'm still happy with the setup."
He just answered, "yes optiplex 620 Pentium D, duo core."
So I think that settles things... do you?
Intel reused the word "Pentium", for their recent budget processors, because there's "Pentium Dual Core" as well, which I believe is actually a Core 2 Duo but with less cache RAM. Pentium was intended for their line of CPUs between "Core 2 Duo" and and the very budget "Celeron" line. But now they've changed again with the "Core i" series, "Core i3", "Core i5" and "Core i7". Not quite sure where the "Pentium D" fits in though, although I've seen it in budget laptops.
Pentium was originally for Intel's 5th generation 586 series CPUs, which came after the 486s. "Pent" implying 5 or 5th generation I think, fake Latin. ...something to do with that they can't trademark numbers, like 386, 486, 586, so they had to give it a name and register it as a trademark.