Would using fallout2HR.exe instead of fallout2.exe or changing the stuff in ddraw.ini affect in any way my using MegaMod?
Yes, it will and it won't. HiRes mod should be ok to use. Changing stuff around in ddraw.ini that comes with Megamod could be trouble if you don't know exactly what you're doing. Except adjusting
[Graphics]
;Set to 0 for 8 bit fullscreen
;Set to 4 for dx9 fullscreen
;Set to 5 for dx9 windowed
;A dx9 mode is required for any graphics related script extender functions to work (i.e. fullscreen shaders)
;Modes 1, 2 and 3 are no longer supported
Mode=4 => 0
;If using a dx9 mode, this changes the resolution
;The graphics are simply stretched to fit the new window; this does _not_ let you see more of the map
GraphicsWidth=640 => your resolution of HiRes mod
GraphicsHeight=480 => your resolution of HiRes mod