It's quite understandible but I don't think that it would be cost effective for an upgrade.Somehow I doubt that a P4 with hyperthreading would work on your motherboard as a guess.
I saw this deal over at newegg that someone else mentioned before on this forum.http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.278844&cm_sp=InfoCenter-_-AMDAthlonIIBundle-_-na
Only thing it was missing was the hard drive,a DVD burner,CPU heat sink/fan and an operating system.
As you can see a new budget build would be better.
Just save up your money and during the meantime study on what you would like to build (come back and give us a budget).
That old system could still be useful as a secondary PC.