Case and power supply question for 2.0 gig pentium 4.

[mellondust]

How can you tell what a processor requires for a power supply. My friend wants me to build him a new computer and wants a 1.9g or 2.0g pentium 4 system. I have noticed as I have looked around that some cases with 300 power supplies at newegg say up to 1.7g or something like that. What is the recomended powersupply for a 2 gig p4. Any suggestions while we are at it on the absolute best case you can buy for around $75 that meets these requirements. Thanks!