well I'm not sure about ALL Dell computers but I've heard that they use psu's that are a certain size and that retail psus won't fit in the cases. Not saying that as 100% fact but have heard that.
Also you need to make sure that all the pins on the processors match that you will be swapping out. Not trying to say you don't know this but intel moved from socket 478 to socket 775 so they may not fit the other motherboards.
For the graphics card in the 4700 what is the budget and We can help more
do you mean all your CPUS are LGA775?? If so, they arent pin driven, the 775 CPUS are a flat Pinless (contact oriented) CPU. If you find this useful please add to my reputation! and good luck on your upgrade!