You can't really "upgrade" this Dell. The P-4 in question is (most likely), a socket 478.
That would be a complete dead end. The DDR memory is a repair part, basically.
So if you're asking, "how could I upgrade it", it doesn't really qualify as being an "upgrade" issue. Anything more than an extra stick of RAM would be a massive waste of time and money.
The only thing usable from this is likely the case. Which is also doubtful, as most OEM Matx cases have very poor ventilation. Depending on chipset, which in the case of socket 478, the best you could hope for would be Intel 845, which still might leave you with all IDE drives, both HDD and optical
So, IMO this should be a, I'd like to build a new computer issue, not an upgrade.
This is the kind of machine you load up with security software and and use to cruise the web.