only thing I can see you might have to contend with would be the difference in wiring of the motherboard. The new case may have different labeled wires or have some using molex connectors and what have you. For a newbie at building, it might be a little confusing, but with help from the people here at CF, we can get you through it.
BTW ... I'd stick with the 450w power supply from the new case. Do you have specs of the motherboard you plan to transfer? i.e. brand/model #. You haven't told us anything about those things ... we may find that your motherboard is incompatible with the power supply connectors or something of the new case.
**edit** I see you listed that you have a slimline pc. Hmmm ... got to make sure it is ATX compatible. You see ... the several connectors on the back ... i.e. ps/2 for keyboard and mouse, usb, etc. ... might not be lined up to be compatible with a typical ATX tower case that you're wanting to transfer to.