It sort of depends; was the old case one of an hp, dell, compaq, etc.? (Prebuilt)
If so, then it may be more difficult, especially if you're going to use the power supply and maybe the motherboard in a dell; they pretty much superglue them to the case lol..
ive replaced the motherboard once before so i assume its not glued lol also t was bought as a custom made cmoputer. I thought id get one with the power supply built in so would that make it easier?
To answer the question of the thread starter, what PSU do you have? if the casing is beatup like you say i wouldnt use the PSU, getting a new case is bassically building a computer, if you have those skills you will be fine.
Putting all of your parts into a new case is a fun experience if you have never done it before. You will find more then enough information on the process posted all over the web.
transfering parts from one case to another is relatively easy as you take note of how the parts are arranged in the original case and you just replicate it in a new case. In short, yes all it is just throwing all the old parts into the new case and hooking up the cables and front panel.
I thought id get one with the power supply built in so would that make it easier?
What you probably mean is to get a case that comes with a power supply. All PSU's can be removed by unscrewing the 4 screws from the back of the case and unplugging everything.