I'm very happy about my dualcore AMD, definitely a step up from single core. I did choose a dualcore with the same per-core GHz rating as my old singlecore, which meant going from a 3500+ to a 4400+...
But I'd rather have a dualcore somewhat-slower-per-core system than a single-core super-fast system. Multitasking is just so much smoother, especially with heavy stuff running in the background. Loving it
Celeron? Ewww, handicapped cache
(but cheap - we do like that).