SDTVs are pretty much equivalent to standard resolution arcade monitors. This means you can run ~240 visible lines progressive or ~480 visible lines interlaced on them. Maximum horizontal resolution is determined by the input bandwidth of the set and tends to be around 720. 320/640 would give you square pixels, which you probabaly want.
Note that TVs are overscanned, so you won't see the whole picture anyway without adjusting things.