It depends
If your 2.6GHz P4 is running on the 400Mhz bus then you'll notice a huge performance boost. If it's running on the 533MHz bus then you'll notice a large performance boost
If it's on the 800MHz bus with HT Technology then you'll still notice a performance boost, although not quite as large as for the other P4 chips
If you want to find out what chip you have download CPU-Z - this will give you all of the information you need on your CPU, memory, motherboard, etc