it is very likely that you can just move the drives from the old system to the new system (or replace the motherboard in the existing system) and when it boots up it will work perfectly with no changes required.
if any changes are required, they will be minimal - almost trivial - tweaks. unless you somehow manage to destroy your drives, you will NOT need to do a full reinstall.
if you're using the stock ubuntu kernel then it should auto-detect the network card and load the appropriate driver.
it will probably autodetect the video card too and load the right driver for X. if you're running it text-only or headless, there's no chance of any problem there.
your drives are sata, with software raid0. again, the kernel should auto-load the right drivers even if the sata chipset has changed.
Overall, it should Just Work.
(of course, it never hurts to backup. it's better to waste a small amount of time than all your data)
I've lost count of the number of times that I've upgraded machines like this. It's usually a very straight-forward, no-fuss procedure.