I didn't think they even made 400Mhz Celerons!
There are celerons from 266MHz all the way up to 3.6GHz.
For the most part a celeron is nothing more than a normal pentium 2/3/4/D with half the onboard cache disabled. (The 'normal' pentium chips have two cache banks on board. However, during manufacturing it's not unusual for one of them to contain a defect. Intel can force disable the bad cache bank, slap a celeron sticker on it, and still sell it for a discount -- beats throwing them away. Because of that, you'll find celeron's at pretty much the same speeds as 'normal' pentium CPU's since they can be identical, other than the cache.
Other than cache size, celeron's are often underclocked as well compared with the 'real' chip -- minor manufacturing defects sometimes only represent themselves when running at high speeds with low tolerances. just slowing things down can sometimes make an unstable chip usable as well -- voila, Celeron.
Anyway -- I bought a P2 400MHz in october 1998, so 9 years ago. I don't think that video card was out by then, though.