Na its ok, just take into account that some people have slowww computers still just ensure everything runs as smooth as possible and that you don't load EVERYTHING at once, as that could be troublesome.
Is that the window size or the entire play area? lol
320x240 fullscreen isn't much different from 320x240 windowed.
but if by fullscreen you mean mazimized to the users current desktop resolution, you maight have trouble, because most people use a minimum of 1024x768 nowadays.
On Windows Vista, my resolution can't go any lower then 640x480 anymore so yeah, all those 320x240 games now have large black areas around them. Best keep with larger resolutions if you plan to have your game played the same by everyone in your target audience.