if you want to image DSOs then the mod for long exposure is a must.
am not sure if the guy in South Australia sells modded TouCams
if not you will have to either do the mod yourself (shudder) or buy a commercially modded Toucam-for example the Atik 1c or higher, or SAC 7 or higher
While admittedly the ETX 90 is not the best for DSO imaging, still there are plenty of people from the "get the most out of your existing scope" school of thought who have imaged all sorts of exotic objects with the ETX 90.
The best example, if you are already not famiiar with his work, is Job Geheniau
http://www.samage.net/~geheniau/astronomy2.html
Nelson Viegas has done some impressive work with an even smaller scope , an ETX 70.( Nelson uses an out of the box ETX 70 with no mods.)
http://www.astrosurf.com/nviegas/
I myself have imaged over two dozen deep space objects with my ETX 70, and even though I have a CG5 Go To mount and 8 inch and 5 inch Newtonian OTAs, still intend to keep on going with my ETX 70-target is 100 DSOs-just for the sheer perverse fun of it. Unfortunately I have no website but I have posted some examples in the ETX section of Cloudy Nights forum
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
and here's my M42 with an ETX 70
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
So you dont HAVE to get a new scope and or mount, at least not just yet.
Remember for autoguiding you are going to need a modified for long exposure webcam or Meade DSI at the least, so why not make that investment in a long exposure webcam type device now, see what you can squeeze out of your existing scope and once you have got the hang of it, then look to upgrade?
Finally, if you do want to get a new mount and scope, I am sure there will be plenty of people who will have views but here's my 10c -suggest you consider getting a -a CG5 or equivalent and an Orion 80 ED as the first step. You could use the ETX optical tube and webcam when you get around to autoguiding
sorry for the long post , but just wanted to point out that the answer need not necessarily be an immediate equipment upgrade