I have a half inch main supply coming from the street (house built in 1954). Is there any benefit if I run 3/4 PEX from the water meter to the various feeds and then branch off 1/2" PEX to the faucets? Does it increase the flow or will it reduce it? will it improve pressure?
Flow resistance, like electrical resistance, increases with conductor length. If you are going a long way (say, over 50 meters) you will get better flow by running a larger pipe. The volume is a non-issue - (static) pressure won't go down even if you install one of those giant green gas storage containers in the yard.
In no case will you get more flow than what the incoming pipe can provide, larger pipes just mean you will not lose part of this flow to resistance.
In a standard house you don't have enough piping (or demand) to make a difference.
Running 3/4" piping from the 1/2" feed will slightly decrease the pressure loss, meaning you'll get more pressure at your fixtures, but the difference will probably be very small. Most of the pressure loss will have already occurred in the 1/2" line before it reaches your house.
So, it will help some, but how much will depend on how long the run is compared to how long that 1/2" service connection is.
Pex and copper have differing inside diameters. Copper has the larger opening. As to 1/2 copper to 3/4 pex I dont know for sure.
This said I would not run my hot lines at the larger size as the pipe holds more water and will waste more water waiting for hot or sitting in the pipes cooling.
If you can insulate both hot and cold pipes... its a minor cost that will pay for it self soon.
There is no use in ever installing any plumbing that is bigger than your main supply line. At best you will have no issues. Worst case scenario is you will lose some pressure based on the increase in volume.
If you want to increase pressure you need to first increase the main supply line. If there is no chance of that happening then decrease as many things as you can to 1/4" (toilets, faucets).
It is true that 3/4 supplies more water and lowers pressure. When faced with this kind of question--resize to the ridiculous (you'll pass many tests with this trick too).
So if you have a 42 foot diameter pipe coming from the river to your house, what will the pressure be? 1 pound per square foot, perhaps? And almost zero resistance, and volume like crazy. Now pretend it is a natural hot water spring you're feeding from. You'll be waiting hours for all the non-moving water that has cooled in the pipe to move through your showerhead while the natural hot water spring refills the 42 foot pipe with fresh HOT water, eventually making its way to your shower.
The answer in short about your original question is, yes, change your mainline, but not to 3/4, instead to 1 inch PEX, this will greatly help flow, and will stop those annoying pressure drops when someone flushes a toilet while you're in the shower. MAKE SURE your hot water supply to the shower is 1/2" and insulated! There is a really good you tube video about this from one of the U.S. plumbing associations: search plumbing layout in you tube, and then look for a bunch of letters such as AAPPA or some such abbreviation as the uploader.
You can have a 2 foot diameter pipe and not lose pressure! As a matter of fact you would be greatly extending your pressure tank capacity greatly. A larger pipe has less surface per volume of water. Going to a larger size would greatly reduce friction and help keep pressure. A half horsepower pump could push water across the United States if not for the friction of the water against the wall of the pipe. Push but not pull