I remember the Yukon FX got a pretty good review in one of the mtb mags (AMB or MBA). It's a great buy for $700, but consider it will be heavier (probably) and require more maintenance than your HT (or compared to an equivalently priced HT). By maintenance I mean regular shock servicing, and probably having to replace shock bushings once a year (about $55).
Depends what sort of riding you're into, for the price of an Alias you could probably get an '08 Talon which would make a great XC bike, do some bargaining if you can find an '08 model in your size as the '09's are now coming out.
Honestly though I probably wouldn't get the Yukon FX unless you plan on getting a better fork for the front at some stage (something like an Air Tora), the Dart just wouldn't do it justice. So in reality if your heart is set on a dually I'd save up and get something better. After all you've started out on a Rincorn, so you're probably ready for the next step - get a $1000 HT or save about $1800 for a decent entry level dually (unless of course you look 2nd hand). In my opinion with the FX you're getting a dually but component quality level probably isn't much better than your current bike. For 2nd hand bikes check out the forums on
www.farkin.net or
www.mtbdirt.com.au if you're in Brizzy.
I would seriously look at the '08 Talon especially if you can get it for a song.
http://www.giant-bicycles.com/en-AU/bikes/mountain/1130/28947/
Otherwise 2nd choice is the '08 or '09 Alias, the '09 has a better fork, the '08 has Sram drivetrain. I brought the '07 Alias for my husband (but he doesn't ride it so it's mine), it's a really decent bike in regards to components, the Dart was a bit stiff for my liking but it did seem to work off road. I already had a Tora 318 Air off my old bike so I put that on it, with that change alone I think it brings it to XTC territory in regards to ride.
Happy shopping. I think it makes sense for you to buy a whole new bike, the Rincorn could be upgraded but to get it up to say Alias / Talon level it would cost a lot more than just buying something new. Your other option would be to pick up a better crank set 2nd hand (off Farkin), to take you through until you can afford something better.