Laura,
Even though you are not a pro(as of yet) and even if you are not striving to be as such,it seems to me that you are taking your hobby seriously,since sound quality is so important to you.
I've been a serious hobbyist for 18 years and what I find to be true more than anything,is that authentic sounds & having the ability to create with them,is what brings the most inspiration.If you're not inspired by the sounds,you can't create.
That being said,you'll need to spend a considerable amount more of money to get the sounds you want.
In accordance with what you want,I can only think of two keyboards that have great sounds & are very easy to use,as far as pro keyboards are concerned.
The Korg TR-61:
Plenty of sounds,multi-sound layering,moderate tweaking capablities,very realistic sounds for the price range and a 16 track midi-recorder to truly sound like an orchestra-without sounding fake.
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If you're not really into the recording aspect of it and you really want to go crazy with creating really unique,out-of-this-world sounds and very easily,then the Clavia Nord Lead 2X is for you.
Both keyboards are $999.00 and it's a big investment on your part,but a very worthwhile & rewarding invesment if you want to get serious about creating.
Even though this is way out of your targeted price range,these keyboards are the cheapest & easiest to work with and they are the most reasonably priced keyboards out there that will give you the sound quality you want,(compared to all that is out there in the professional line).
The best advice I can give you,is to puruse Sweetwater.com and take a good look around thier site.
Also,it would be in your best interest to speak with my sales rep: Yuval Fuchs(ext.1367),or speak to anyone in the keyboard dept as the sales staff is the most knowledge people in the business(I have found).
Also,they have an amazing return policy & product support.
If you live outside of Indiana,then you will also have the benefit of no sales tax(out-of-state tax exemption).
Cheers,
Elwood