A Celeron CPU is typically a more cost effective version of the Pentium CPU.
They have been based on Pentium 2s, 3s and 4s [and soon the Pentium Ms]. There has never been any Pentium, let alone Celeron CPUs retailing with a clock speed of 4.0GHz clock speed.
Typically a Celeron will perform worse [not always, the Celeron 300A was superior to the P2-300 in cases where the smaller but faster cache was relevant] USUALLY due to a lower front-side bus speed, a smaller [and usually with inferior associativity] cache and fewer features.
But I'd put the latest Celeron Ds versus any pre-Northwood P4 anyday. Don't get taken in by marketing. If I can save 50% of the cost of a 3.2GHz Presscott by using a Celery clocked at the same speed, I lose a tad of RAM bandwidth but I can now upgrade from a 6600GT to a 6800GT. Which do you think is going to matter more?
Today's Pentium, tomorrow's Celeron basically.
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Sorry for the long post, ran into this idiot CRM manager [from Sales naturally] who kept insisting on P4s for his workstation [he actually wanted us to buy $1000 Northwood P4 2.8s over $650 Celeron Ds, everything else being identical] because he 'knew' that a Celeron was only equal to a P4 running at half the clock speed of a Celeron.
Oh yeah, 512mb system RAM... since it's using 'superior' shared RAM integrated graphics, is better than a 256mb Quaddro. Yep yep.