That combo should fit your case, unless it's really old. All micro atx boards will work in either micro atx or full size atx cases. Newegg sometimes has some rosewill cases with free shipping, as low as $20 with their email promos.
I don't think a happy meal box will be big enough though I am from the UK and you do things bigger in the US (especially in food). The IDE connector is what the old PC would use to connect to your hard disk and optical drive (CD/DVD) you can connect both on 1 cable but its slower. Current power supplies fit and current motherboard but im not sure about older Dell ones, also it may not supply enough power anyway (I have no idea but check before you try it). When you look into it you may find everything from the old pc can be upgraded very cheaply (individually) and you better spending an extra few hundred dollars and making a total new build maybe keeping just the DVD drive.