That's a pretty substantial upgrade, although the difference will depend on what you're using the PC for.
If you're just browsing the web, word processing, etc. then you won't really notice a difference. On the other hand, if you are playing current games (FEAR, Battlefield 2, etc.) or some other things that put a high load on your CPU, there will be a rather large increase in performance.
In technical terms, the 1.69GHz CPU can theoretically process up to 1,690,000,000 cycles per second. The 2.4GHz CPU can process up to 2,400,000,000 cycles per second. That's a pretty big increase for applications that are going to be pushing your hardware to the limit.