What is the best power to view larger galaxies and nebulae?

Well, it depends on what nebulae your looking at. If you want a cool but cheap option, just get maybe a 4mm lense, and a few filters. Filters really hep bring out nebulae, but you have to get the right ones. Oxygen filters are probably the most expensive, but very impressive. Filters will bring great contrast between space and the object your looking at, which is really cool.

To much power isn't good for viewing the objects you want to view, I use a 36 or 40 mm eye peace along with a 2X barlow lines.
I use a 12" LX200 with the eye peaces I suggested, they work great for me.

Magnification is not a major factor. The major factor in spotting galaxies is aperture, and your 90mm scope is just not going to show you much beyond a few of the brightest galaxies. This is the fallacy of inexpensive computerized scopes like yours: all the money goes into the computer, and very little goes into the optics. While its computer will point it at all sorts of things, most of them can't actually be seen with that small a scope. I traded my ETX90 in on a 10" Dobsonian reflector, and it was one of the best moves I ever made.

[Edit] Both the above answers are wrong in opposite directions. You cetainly don't want to get a 4mm eyepiec, as that would give you a ridiculously high magnification. Filters would cut your already limited light even more. A 40mm or 36mm eyepiece won't help either, as it would be choked by your scopes 1.25" focuser. The best eyepiece would be an Orion Stratus or Baader Hyperion 24mm. This will give you a slightly higher magnification, but a much wider field of view because of the eyepiece's design. But really an ETX90 is mainly good for the Moon, planets, double stars, and star clusters.

Size has little to do with brightness.  M31 and M101 are both large but M31 is naked eye bright and M101 tough to see unless under dark skies.  Both work best with low power eyepieces which for me is a 35mm Panoptic.  Same goes for nebulae.

Unfortunately, a 90mm telescope is not suited to deep-sky objects like nebulae and galaxies. A telescope of that aperture is better suited for bright objects (planets, the Moon, the Sun with appropriate filters).

Magnification is irrelevant. Remember, magnification is just that - changing the *size* of the image. It does NOT affect the *quality* of the image. For example, if the image is fuzzy, magnifying it only makes it a bigger fuzzy image. If the image looked clear enough with a long focal length eyepiece but then seems fuzzier when you switch to a shorter focal length eyepiece, it is because you are magnifying the already-present imperfections in the image (such as atmospheric turbulence, etc.).

The three main functions of a telescope are (1) to gather as much light as possible, (2) resolve fine details, and (3) magnify. Unfortunately, many people who are new to stargazing fall victim to promises of "high power" because they are unfamiliar with what the telescopes main functions are. When purchasing a telescope, you generally want the largest aperture possible on your budget (although admittedly there are other considerations if you are doing specialized work). The ability to resolve details is also a function of the aperture size so with a larger aperture comes higher resolving power. Magnification depends only on the focal lengths of the primary mirror or lens and that of the eyepiece, having nothing to do with the aperture size. So, for faint objects you want a large aperture telescope.

I teach observing courses for nonscientists and although we have an ETX, it is almost never used because of the small aperture. We rely almost exclusively on non-computerized Dobsonian reflectors of 6, 8, 10, and 12-inch apertures because of their superior light-gathering abilities and ease of use. In fact, we use the ETX only once per term, in an exercise to demonstrate the limitations of such a small aperture.

Don't be discouraged, though. You can still learn to use the ETX-90 to your advantage and improve your observing skills. The aperture isn't much larger than some binoculars so try one of the books written for binocular observing; there are many good ones out there and some try to get you to deep-sky objects so you may be interested in that. Of if you really want to upgrade, look for a retailer that accepts consignments or trade-ins (there are more than you think!).

_Sky & Telescope_ magazine has a good set of resources for beginners including advice on instruments as well as using what you've already got.
http://www.skyandtelescope.com/letsgo/toolsforstargazing

Either way, good luck and keep looking up! Clear skies to you!