Upgrade Motherboard

I would like to upgrade a Dell system with a new motherboard combo. It's a home system media mostly and no gaming. Budget system basically and would like to use some legacy boards mostly PCI. Am running Windows 7 on a Pentium 4 currently with 1gb memory. Not bad really for basics. Too slow of course for most media or audio.

What area of motherboards/CPU chipset combo should I be looking at? ATX form factor I expect. Built many system from scratch in the past. Only using the Dell for the case design which I like. Have some Antec case system but I like the modular design and easy access of the Dell cases. I know they are proprietary motherboards and no docs but an ATX form factor should work.