You should launch the task manager, then run the game with the selected resolution, reproduce the lags, then minimize it and check the CPU/RAM usage.
Big RRs can eat up a lot of RAM. Even if a PNG map is about 1 or 2 MB big, it is compressed, and must fit in memory uncompressed (which can be 100, 200 MB or more). This is why the "Map Memory Warning" slider is present in Advanced settings, to let you know when an uncompressed map is going to hit the threshold (you can set it to "Always" so it'd always display you the amount of RAM it'd be using). It does look like one of the issues you're having here.
Also try to switch between Direct3D 9 (shader) and Direct3D9 (CPU) modes. The shader one should always be preferable, however, but I haven't tried it on systems like yours. The D3D9Wnd module only works with these two modes.