personally, if you are using only mild or standard cams and are aiming for anything from standard to under 350 - 400 break I wouldn't bother getting the head flowed, especially if you intend to build a car for the road. You would be better off with new valve guides and getting your old (or new) valves lapped in properly.
If on the other hand you intend to build a track car with in excess of 400 break, wild cams and big turbo, there are 2 stages you could take. Don't even do the first stage if you feel you haven't got the skill or the tools. One of the most important things is the having the space and ability to clean everything up so it's spotless, after you've done the work, an industrial sized parts cleaner and compressor with a blow gun would be best.
First stage (won't do very much in real terms but it will make you feel better :-D ) would be to remove & number all the valves then by hand just lightly clean up the combustion chambers with scotchbrite and 400 W&D, removing any sharp edges or casting marks without taking out to much material, paying particular attention to the areas around the valve seats both inside the combustion camber and just inside the port (being very careful not to mark the valve seat itself). Don't bother touching the inlet ports but clean up the exhaust ports in the same way you did the combustion chamber, just removing any casting marks. Then once everything is how you want it clean it all up.
Once it's all clean and dry you can start lapping the valves in with fine lapping paste. Then remove all the valves again clean it all up again and assemble, putting it all back in the order you took it apart in.
The other option is taking your complete head with the new cams, valves an springs you plan on using to one of the specialists listed above and asking them to rebuild your head with new valve guides and seats to match your new valves, and being prepared to part with the best part of £600 - £700.
I have (but I can't remember who I used for the life of me) done both options in the past and the second option does make a difference on the right engine spec.
To answer your last question, on the second option provided you got he whole head rebuilt and ported/flowed by a specialist I would estimate you would gain another 10 – 15 bhp possibly more but it does depend on sooooooo many factors, what engine it is and the spec of that engine.
Like I said, if it’s a road car I wouldn’t bother unless you have deep pockets.
