If you do it ourself, be careful to make sure that the power supply is properly connected, both to the motherboard and to the power switch. Power supplies have two keyed connectors attaching them to the motherboard. The motherboard also should be connected to the case with at least two spacers that screw down in addition to all the plastic posts that will be in the case kit. This insures that cards fit properly, and keeps the board stable. If you buy from a show vendor, make sure they test it in front of you.
Any reputable vendor has their own RAM tester.
Generally, there is no real price difference between conventional fast-page RAM and the slightly faster EDO variety, but make sure your motherboard uses the type of RAM you're buying. Most better motherboards will happily auto-detect the type of RAM you use and configure themselves correctly. But you can't mix, so make sure you only install one type, whatever that is. I have not yet tried it in a system, so I cannot tell you whether or not that is so. There's also the IDE-SCSI decision.