Using hires (640px) uses twice the bandwidth of low res. Upping the color depth is proportionally less of a hit (from 4bit (16col) to 6bit (ehb) color is only a 50% jump (technically a little less, but for conversations sake)). The higher color depths (32colours through to HAM) are available in interlace modes though (320x400/512).
Additionally hardware sprites use colour registers 16 through 31, so even in 2-16 color modes regardless of resolution hardware sprites will use thier own colours.
There's no reason a stock ocs machine couldnt do games using higher resolutions, but being that required bandwidth at least doubles and moving gfx are twice the size (assuming same dimensions onscreen as a low res games) there'd typically have to be less moving gfx (ergo arcade style games arent really options). Put simply a game would have to be written and styled around these restrictions. RAM speed does play a part, but the amount of ram also comes into play here (bigger higher resolutions gfx require more RAM).
As for ECS being able to use super high res (1280px) in HAM, Im somewhat sceptical about that. So far as I know the super high res modes are restricted to 2bit colour for ECS. Feel free to correct me if Im wrong though someone.
