Hi newcomer and welcome to the Tom's hardware forum.
The 775 socket was a very good socket few years ago, but today has been replace buy the 1156 and 1366. (i3, i5 and i7).
Now, u need bear in mind this:
1- Price of the processor that you want install, some 775 processor are more expensive that the i7, but the i7 is the best processor.
2- What socket type is the processor that you want install?
3- What is the main use that you want for the rig? games, surf the web, video edition...
4- Do U want OC the processor, GPU and RAM?
Some 775 sockets support DDR3, but are more expensive that a 1156 or 1366 mobo socket.