You don't need a special case for the P4 necessarily. Only if you buy the Intel motherboard. There are 2 compatibility issues for the P4 and your case:
1) The power supply. You need the new ATX spec with the extra 12v connector for the motherboard. If you get the case without the power supply, just order another power supply that is compatible.
2) The heatsink is supposed to connect to the motherboard tray, but this is not a requirement (unless you buy the Intel P4 motherboard). Instead of clipping to the processor socket, the heatsink is supposed to anchor to the motherboard tray through holes in the motherboard. However, most motherboards have a way around this. The Asus motherboard comes with a special backplate that you attach the motherboard to, and the backplate attaches to the motherboard tray. I have the Abit motherboard and they simply have an anchor that attaches to the motherboard itself. You wouldn't want to attach a massive heatsink to it, but there aren't really any sizeable heatsinks out there yet (most of them can handle being attached to the socket anyway).
Check the size of the motherboard you plan to buy and simply buy a case that will fit the dimensions, and then check the power supply to make sure it is P4 compatible. If you are worried about the heatsink connection, a lot of case manufacturers are offering to drill the motherboard tray for an additional fee (I know Addtronics will do this for $10).
I had a rather large case to begin with (Addtronics 7896A), but it isn't a "P4 compatible" motherboard and yet I had no problems installing a P4 into it.