What you need to determine is what CPU it will support (i.e. what socket does it have), what sort of RAM it uses, what GPU it would support, and what sort of PSU it needs. If it really is from the 90s then it will only support components from that era, which will be harder to get and probably more expensive than modern ones.
For any sort of useful upgrade you will need a new case, motherboard, processor, RAM, GPU, hard disk, and optical drive. The power cord might still be good for a few years.