hi! i have a Dell precision T3400 with Core 2 Duo processor. i want to change this processor with core i7 3rd gen, Can i change it's motherboard with LGA1155?
If i can change then which One. ATX or Micro ATX????
I don't think you can swap CPUs - C2D is Socket 755, while i7-3xxx is 1155. I've seen some people putting Xeons here, with small socket mod.
Whether you can change the motherboards - check your existing motherboard and its connection to the case. Many Dell desktops have proprietary front-panel connectors, which makes motherboard swap difficult.
And last but not least - that Dell (and its PSU) are at least ten years old. If you want new motherboard, you'll want also new GPU, hence new PSU.
You'll have to do some extensive research to see if there is a dell motherboard that will fit and work. While Dell does use some standard sizes and form factors, they aren't typically industry standard and even Dell changes them enough between several generations that they may not work without modifications.