Typically the 750Ti requires a psu of 400w
The 750Ti is a rather small gpu and should have no problems fitting in the case you have now.
If you really do want to change, the motherboard is listed as a micro-atx, so any case that size rating or larger is usually sufficient. http://pcpartpicker.com/part/zalman-power-supply-zm500le
is the cheapest I could find at this time