I’ve done lots of pushing of 400 to 800 using Kodak Tri-X B&W. I found that an increase in development time of 20-25% worked excellent for me. You might consider doing some test rolls to nail your times down. You can expect some increase in graininess, but it’s really not too bad.
There are tons of variables (film speed, development chemicals…etc.)…when in doubt, test, test, test.