Yeah, the 800 -> 1333 upgrade is not going to make a big difference.
However, the Cedar Mill (Pentium4D) to Conroe (C2D) upgrade is freaking HUGE. A 1.8GHz C2D with 2MB cache can run circles around a 3.6GHz P4D. You're getting a 3.0GHz C2D with 4MB cache. Hypothetically, if the P4s existed to this date, then we're talking about the equivalent performance of a ~6-7GHz Cedar Mill. The difference *is* really that huge. Despite this, as mkreku said, you probably won't perceive this in a lot of real-world scenarios, where you'll likely be bottlenecked by memory latency, HDD and GPU speed.