Any case that is AMD, Pentium ready can be used, best to check that the case has the required PCI slots for the motherboard.
As for the lack of voltage, where from, the Power Supply or on the motherboard itself.
If its voltage on the motherboard, then the manufacurer may have some monitoring software available.
Goto MSI website and download PC Alert4, this will show you:
CPU and System Temp
CPU, SYS and PSU fan speed, if installed
Depending on how much load you have on the PSU, ie scanners, printers, pci cards connected to the PC Power loom, this will affect the out put from the PSU.
I would look to use at least a 350W type, remember if you use a P4 CPU, make sure that the PSU is P4 ready and get one with at least 2 fans.
One final point, look to get a case that has enough fan ports to help keep it cool inside.
Hope this helps.