Hiya BioQuack and welcome to the forum
As far as I'm aware (and anyone else feel free to correct me if I'm wrong) the best processor you can get is a higher 9xx series Pentium D. That chipset doesn't support any of the Core 2 processors.
Edit: There might be some slightly better Pentium dual cores you can use... Bear with me, I'll go check.
Edit again: I looks like I may have been wrong... This link on Intel
suggests differently but with your machine being a Dell, it would very much depend on the BIOS support and also other factors.
LAST EDIT: Ok, after a bit of digging, looks like someone with the same motherboard as you asked the same question as received this answer