Recommendations for desgining optimal power supply

Hi, I am developing a system which will consist of arduino mega, a quectel m95 GSM/GPRS module, a 4 channel relay board, an SD card module, an RTC module, & an LCD. I would need to provide atleast 7V at Vin of mega, 4V to GSM module with around 2A of current supply, 5V to relay board, LCD, RTC & SD card module. I want to know how can i go about designing a power board so that each component gets its required power. I am planning to use a 12V/3A adapter or a 5V /3A adapter. any suggestions?

No, once an Arduino is connected to other devices which require power, the “Vin” or “Barrel Jack” is not appropriate.

When you have a properly regulated 5 V supply, connect it to “5V” and ground. It may be advisable to disconnect the other devices while you connect it via the USB port to program it.

The GSM module will need a supply upto 2+A at the peaks that's happy for a very bursty load.
GSM devices are designed for single LiPo cell, 3.7V nominal with good current handling. A DC-DC
converter well able to handle 2 or 3A, and happy with intermittent loads, might be feasible. Adding
extra decoupling on the output of this might be good to help smooth over the transients.

Using 12V on the input isn't a problem with DC-DC converters, but will mean more
waste heat to get rid of with linear voltage regulators. Perhaps another DC-DC converter for 5V as well as
the one for the GSM module? The cheap units using LM2596's on eBay are adjustable voltage and I've
used a few of them successfully, but not sure if they really can handle >2A...