|We certainly didn't need all that. Most OEM manufacturers deliberately use a barebones BIOS to prevent too much user tampering.|
To answer your question - the XPS 400 is NOT overclockable, at least not thru the BIOS. There are software programs that may work, but for the most part, they're just a waste of time.
Upgrading the crappy Pentium D to a Core 2 Duo is out of the question due to the outdated motherboard/chipset. It appears your system is pretty much maxed out. If you're not happy with the performance, it's time to buy or build something more modern.