I honestly do not know much about computers, and I am wondering about the difference between Celeron and Pentium 4 processors. I am looking at possibly getting a new computer, and I am looking over the sales that they have tomorrow (ah, the fun of day after Thanksgiving shopping!

). I notice that they are all in the 2.5+ GHz range, so cannot see any difference there. But what is the real difference between the two?
I was looking at the Best Buy flyer in today's paper, and they have a Intel Celeron 2.6 GHz w/ 80 Gig hard drive and CD-RW/DVD combo drive for $449.99 with $250.00 mail-in rebate for a total of $199.99 after rebate (Friday until noon only). Considering that I am running on a 933 MHz Pentium 3, this is rather attractive. But if Celeron is garbage, then I'll pass on it.
As a side question, how are Maxtor hard drives? They have a 120 GB hard drive for $139.99 with $90.00 in rebates (total $49.99 after rebates), but it is a Maxtor. I have an 80 GB Seagate, and can swear by Seagate (recommended by a friend, and have not been disappointed), so would not change unless I can count on the Maxtor to hold up like the Seagate. If I buy the Celeron listed above, I will switch out the hard drive that comes with it for the Seagate (I'm slowly getting together the components for a secondary system as I replace stuff, so the extra 80 GB hard drive would go in there).
Please enlighten a relative neophyte about these questions, as I do not want to end up in line at 4:00 AM to get into Best Buy just to be disappointed.