I have the Intel D845EBG2 Socket 478 motherboard. This mobo supports 533 MHz. I am currently running the 2.26 GHz CPU.
You're probably wasting money trying to upgrade just the processor - someone would have to actually GIVE you the P4 3.06 for it to be worth bothering with
The 3.06 GHz Pentium 4 northwood is about as powerful as an Intel Atom D510. Compared to even a $50 processor, it is a joke
You could buy a motherboard, processor (with integrated graphics) and 4GB of RAM for under $120 from the 'egg.http://www.newegg.com/Product/Product.aspx?Item=N82E16813128530http://www.newegg.com/Product/Product.aspx?Item=N82E16819106013http://www.newegg.com/Product/Product.aspx?Item=N82E16820576003
If you don't need the integrated graphics to do any actual 3D, then you could get a better processor by going with an Intel Celeron G530 which is a faster CPU than AMD option.
Either way, you'd be embracing DirectX 10, 64-bit compatibility and the ability to address more than 3GB of RAM.