In your position, i’d be tempted to keep the computer i’ve got. The Celeron’s small L2 cache makes it a considerably slower and less powerful processor than the Pentium. I’m not sure of how a 1.5GHz P4 would stack up against, say, a 2.8GHz Celeron, but in reports i’ve read comparing the two processors the difference in cache size seems quite often to be more important than the nominal GHz difference. I’m sure someone who knows more about this stuff will be along soon to give more details.
One thing i’m curious about, given the current absurdly low prices of mid-range P4-based computers, is why your company would even bother with a Celeron processor in a desktop computer. At Dell right now, for example, you can get a P4 2.8GHz computer, 256Mb of memory, and an 80 gig hard drive for under $400. Beef up the memory a bit, and you’ve still got a perfectly decent Pentium system for under $500, and it will kick the ass of a Celeron.
Of course, what asterion says is completely true—for most basic apps a Celeron will be just fine. My wife has a Celeron-based laptop, and while it’s not the fastest beast in the world, it does fine for what she needs to do, which includes web browsing, email, basic word processing stuff, and reading acrobat files. She can have Firefox, Word, Acrobat, and Thunderbird open all together, and the computer works just fine.
I, on the other hand, sometimes have all that stuff open, as well as some combination of Photoshop, Dreamweaver, various media players, and even video processing software like TMpegEnc. If i’m doing heavy processing work in Photoshop, or video encoding, even my P4 3.0GHz with 1Gb of RAM sometimes starts to struggle a bit, and there’s no way i could do that stuff at any decent speed with a Celeron. So it does depend on what you’ll be using the computer for.