Simply, no it is not worth it.
$400 dollars will get you a cheap Core 2 Duo processor, and a new mobo. RDRAM has been selling for big bucks on ebay so selling that should cover the cost of new DDR2 memory.
Even the cheapest Core 2 Duo will smoke a P4 3.06. And they overclock on stock voltage very well.
Don't think you can mix the memory types but I'm not sure.