But if its a Time machine, probably very little, and almost certainly, for the minimal performance gain you will get, it will not be cost effective.
The reasons for this are;
Time use proprietary parts in many cases, which means that they work as configured, but do not allow an up-grade path.
It is likely you will also find a minimum specification Power Supply, which itself will limit any possible options.
Your best bet, is to keep your monitor, keyboard , mouse etc, and build a new "case only" machine. So you get a new case and power supply, motherboard, processor, and memory, but use your existing CD/RW, floppy and so on, to complete the new machine.
Overall, you will end up with a far superior machine, at minimal cost.