Dell Optiplex 745 to GX620 Please Help

 the optiplex got the orange light, which means that it wont turn up. That happened because my ups was damaged and i needed to replace the battery in it. 

Originally, i was going for a amd radeon hd 5670 or a gt 240, a core 2 duo e6700 and 3 or 4 gb of ram. To fix the issue i handed the pc to a store to fix it. the motherboard was the issue. then, a week later, I go turn on the pc, everthings ok  BUT, The bios and the motherboard are of a gx 620. 

It says in the bios, Dell Optiplex gx 620 bios revision a11. now i have so much issues that i cannot count LOL. the first one is that these weak pentium 4 and pentium d waste very much energy and its going to be very much for the pc to handle. it has a 305w power supply and i runned a power supply calculator and it exceeded the 305w limit sigh... why cant these pcs support core 2 duos.... 

the second one are the drivers. in reality this pc is a optiplex 745. but the motherboard and the bios are from a gx 620. and on top of that, when i buyed it, it haved windows 7 and it has 1gb of ram which causes it to go slooooooooooooooow. the drivers of the gx 620 are only for windows xp and as you may know windows xp drivers are only for windows xp and even if you run it with compability mode it wont work. 

luckly, i have windows xp on a cd, and i certainly know that it will run faster with windows xp. i arrange some pictures for you to see it. and in the back, the pc says that is a gx 620. it has a seal that says that. maybe the person misunderstanded it. Because of the drivers, i have no sound, the video card doesnt support my monitor, 1366x768. should i install windows xp, yeah, i think so.  http://extreme.outervision.com/PSUEngine that was the calculator i used. 

first i used neweggs calculator later that one. it could be that neweggs calculator uses the 130w pentium d processors. when i used the one on the link i used the pentium d 820 3.00ghz and a geforce gt 220 affordable and good and 2 sticks of ddr2 ram plus 2 80mm fans. i dont know how many fans does this thing have but, well....