Personally, I thought the 2008 and 2009 Gary Fisher Wahoo http://fisherbikes.com/bike/archivemodel/457 was the best bike in that range, but since the branding re-org at Trek it is out of production. The bike you are on now has a very upright riding position, and was designed to appeal to novice mountain bikers and people who were likely to spend a as much time riding bike path as actual trail. The biggest difference you are going to find between it, and say the above Fisher, or the Trek 6000 series will be the bike’s geometry. Higher-end hardtail mountain bikes will, as a rule, have a more aggressive riding position, meaning you will be leaning forward more. This will require better hamstring flexibility and strength to be comfortable and efficient, as well as force you to shift your weight back more on descents to avoid endo’ing.
Your other option is to look a full suspension bikes, but in all honesty, unless you can find a ridiculous deal, any full suspension bike that retails for less than 1,200 bucks new is going to be a disappointment, because one or more serious corners will have been cut in order to get the price any lower. That said, the world of full suspension bikes is practically limitless – a "cross country" bike – traditionally in the 3-4"/80-100mm front and rear travel – will ride like the hard tails described above. A "trail" bike, often in the 4-5"/100mm-130mmm range, will be more upright, have a longer wheelbase, and a shallower headtube angle, making it much better descending and more comfortable to ride.
You could consider trying to find a 29" hardtail in your price range. New, they tend to be significantly more expensive and heavier than similarly equipped 26" bikes, primarily because the market will allow it (a Gary Fisher Marlin, for instance, has about the sames parts as your 4300, but costs about $100 more). However, the difference in traction and control over your 4300 will be immediately noticeable.
I would strongly dissuade you from spending serious money on upgrades to your 4300. It is more cost effective to simply get a new bike, there is almost no resale value for the parts you are replacing, and you will have a hard time finding a market for an upgraded 4300, when you will most likely have little problem getting a good price for your bike as-is. If you are going to upgrade the bike, the biggest bang for the buck is brakes. It sounds completely counterintuitive, but you will ride faster, crash less, and have more fun with a quality set of V-brakes (XTR, etc), or if you are on a bike already running discs or have disc-ready wheels, a mid-tier set of hydraulics (Juicy 7s, Lousie BATs, Shimano LX, etc).
Hope this helps
stack