Different programs will stress different components. For example, in high resolution gaming, the video card is usually the bottleneck, even with a 7800GTX. With video editing, it's usually the processor.
For word, internet, etc..., there's really no bottleneck - these tasks won't be using anywhere enough resources to cause a performance hit.
The 3500+ is a great processor for its price, or if you're looking for something that will tackle video editing a bit better, the X2 3800+ is great (but more expensive). For the other tasks you've mentioned, even a cheap Sempron would suffice.