When it comes to observing deep-sky objects (the preferred term), the larger the aperture (i.e., the diameter of the primary mirror or objective lens), the better. A large aperture provides greater light grasp, as well as resolution, which is the ability to distinguish fine detail.
Here are some reviews of the Orion XT8:
http://www.bpccs.com/lcas/Articles/xt8.htm
http://www.scopereviews.com/page1j.html#4
http://www.cloudynights.com/item.php?item_id=1043
http://www.eastvalleyastronomy.org/reviews/skyquestxt8.html
Zhumell is another brand that is worthy of consideration.
http://www.zhumell.com/specialty/zhumell-telescopes-dobsonian.html
Expensive apochromatic refractors provide the most pleasing images of any telescope design. Far less costly achromats work well enough on most celestial objects but can be prone to chromatic aberration or false color on bright ones, depending upon the relationship of aperture to focal ratio. The Orion refractor, since it has only a 90mm objective and a "slow" focal ratio of f/10, should perform reasonably well.
Here are a few sites that you may want to consult.
http://starizona.com/acb/basics/equipmentbasics.aspx
http://www.synapticsystems.com/sky/scopes/telbasic.html
http://www.astronomy.com/asy/default.aspx?c=ss&id=9
http://skyandtelescope.com/howto/scopes/article_241_1.asp
http://www.skynewsmagazine.com/pages/telescope_tips.html
http://www.scopereviews.com/begin.html
http://www.company7.com/library/begin.html
Dave Mitsky