Presumably you are using WinXP on this Dimension 4700 and even with
the memory upgrade there's no guarantee that Windows 7 will work on it
either, never mind Windows 8 !! BTW - Secure Boot is a hardware thing
and even if everything else was compatible this would never be able to
work on your Dell. Secure Boot needs the new UEFI replacement for the
venerable BIOS Chip - not something you can retrofit to an old PC.

What would work is to use Ubuntu on it as the only OS then put Virtual
Box on and install your WinXP that came with the Dell into that for
those few things that still need Windows. It's how I use the few
programs I still need that only work in Windows and does it very well
too. Not only that, you won't have to re-boot to be able to run those
Windows programs either <lol>

Something to consider to be able to stop having to pay out for the
next version of Windows every few years ? Also bearing in mind that
often the next Windows also needs the hardware upgrading too !!