I've used both nvidia and ati, and both were about the same. Ati seemed to have better image quality overall, although since neither of my cards are one of the more recent ones, I would not know if nvidia has addressed that issue or not. One problem that I did have with the nvidia card though was that after a year and a half of usage, some of the capacitors began to leak and burn out...
It also depends on what you are going to use the card for. Games, Nvidia might be slightly ahead of Ati; its opengl is and always has been better than ati's; however, if you want to run games with Anti-Aliasing on, ati quickly pulls ahead of Nvidia. For graphics design, Ati has better 2-d support and in my experience better quality.
All of this also depends on how much you are willing to spend on the card, and what your current hardware configuration is. There's no point in getting a 500$ video card if you have a slow processor or low memory, since that will just bottleneck your system and not allow you to use the full power of the graphics card, resulting in a waste of money (I'm not sure about that celeron of your's... if I were to get a high end gfx card i'd upgrade that to a p4, or even better to a p4 with ht, or just as good, an amd 64). And as people have mentioned before, always check for compatabiliy before you buy the card.