1) most powersupplies that come in cases are crappy (unless its an antec case). so most of the time its better to just buy a seperate power supply.
2) for a 6800 card probably a minimum of 450 watt is in order..
stick to the major brands: antec, OCZ, FSP fortron, enermax, etc..
something like this: http://www.newegg.com/Product/Produc...82E16817104954
3) any mid-tower or full-tower ATX case will fit.
4) different generations of processors fit on different socket motherboards. older pentium 4s ran on socket 478; newer pentiums run on socket LGA775. newest core 2 duo intel processors run on only certain LGA775 motherboards. same thing goes for AMD, their latest socket is AM2.