You are already achieving a refresh rate of greater than 30Hz. If your computer's control panel only shows "30Hz," it is telling you the refresh rate in terms of frames. It is impossible to change this on the display you are using at that resolution. When using 1080i, you are interlacing frames, which results in fields. The very 30Hz frames you are seeing are being drawn at 60Hz in the form of fields. In the computer gamer's ideal world this means you are losing some information; this perspective does not take into account the purpose of interlacing, however, which is to provide smoother animation effect from trickery of persistence of vision, while eliminating half the bandwidth required to achieve the effect. If you really want "60Hz" in terms of proper, full-frame animation, you cannot achieve it with 1080i.
It gets trickier when attempting to compare with 720p on a CRT. Ordinarily, you might be surprised to find a nicer effect of animation and higher detail with the full-frame animation of 720p running at 60fps--and at 60Hz. But as consumer CRT HDTVs are always interlacing at "HD resolutions," i.e. both 1080i and 720p, with 720p undergoing some processing to convert to a displayable mode (which is almost always interlaced--1080i again, or possibly 540p), you might not see a drastic difference between these two modes on a CRT HDTV. Only 480p will be absolutely progressive on these kinds of sets without question. But there is still no reason not to eyeball it and see if you prefer one mode to the other. If you find one "HD" mode preferable to the other then it is probably because it is better suited to that particular display.