I dug into this for you. I checked the forums at Dell support, and one of the posts asked the same question as you. The fastest the dell guy said was a 3.6 ghz p4 of the same era as the chip you have.
Heres the link:http://en.community.dell.com/forums/t/19288313.aspx?c=us&l=en&cs=19&s=dhs
and this one for more info. They got a 3.8 ghz chip and also a p4 670 to work.http://en.community.dell.com/forums/t/18706191.aspx?c=us&l=en&cs=19&s=dhs
May be cheaper to bite the bullet and get a new rig. Lots of deals right now.