hahahaha OK there goes my theories, I was assuming average use.
1. Video Encoding(DVD--> DivX rips, this really takes a while on my celeron) - HUGELY CPU intensive a Duron at 1Ghz will smoke a Cel 550.
2. 3d Graphics rendering(Bryce 3D, 3D Studio MAX, etc) - Also fairly heavy on the CPU especially FPU, though this kind of stuff needs hard drive speed to, unless they are really small animations.
3. 2d Graphics rendering(Photoshop, PSP, etc) - Pretty much the same as above.
4. E-mail, Net surfing, Downloding etc.(basic stuff, napster, ICQ etc) This won't really matter...this kind of stuff wasn't CPU bottlenecked on my K6-2/350. It'll be a bit faster but you won't notice worlds of difference. I'm doing this stuff on a Pentium 166 (might be mmx..not sure) with 80MB of RAM at work. Stuff takes a while to open, but once it's open it's speed limited by me and my internet connection (a T3 to my office building I think). (I know P166 is BBBAAAAADDD, I get a new PC in january..probably a P3/733)
5. Games(mostly UT, NFS(all of them), FFVII, Starcraft) UT is very CPU intensive...I was looking at Anand's review history today, I want to see how much I can net overclocking my 700Mhz TBird. At 1024x768x32 with a GeForce2 GTS, upping a TBird 100Mhz gains about 3fps (this was from 700 to 800 to 900 to 1000, each was about a 3fps jump). So your Cel 550 to Duron 1000 should give you a pretty significant jump becuase a Duron is faster clock for clock than a Celeron and that's about 15fps clock speed difference. Granted your video card is slower than a GTS but still...UT is very CPU intensive. Quake3 on the other hand anything from about an Athlon 500 to an Athlon 1000 performed within 2fps of each other on Q3A at 1024x768. FFVII runs on a playstation...I doubt there's a bottleneck in your system, Starcraft ran on my P200MMX there's definately not a bottleneck in your system. Not sure on NFS...but some games should definately see improvement.
Overall for what you do it's definately worth it.