I need some camera/photographer's opinions. Professional's opinion to a beginner's?

I have a Nikon D40, currently. I know it's quite cheap and very low quality. I really want to get into Photography. I'm definitely a beginner and I'd need some help. I want to sell my D40 and get a newer and more up-to-date camera that'll really help me with the Photography goal. If anyone can give me any advice, that'd be greatly appreciated.

Also, my upfront question is: is how much could I get for my D40, and which camera would best suit my beginners need for photography. Any lenses with that? I have a budget of approx. $1500. I know it's not all about the camera, but I'll need one to take pictures, right? (:

Again, any advice to a beginner would be amazing. Thanks in advance.

Added (1). I love Nikon and Canon. It doesn't make me any difference whichever you recommend. What are your opinions about that as well?

Your D-40 has a pixel count still good in todays market and numbers. However, get a body that can accept older manual Nikon lenses, like a D-200, D-300, D-700, D-90 and so forth. Just make sure it can take a AI'ed manual lens and meter couple with it. Ask. You will understand later.
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This is something Canon can't do!
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Look on E-bay, Amazon.com or Craigslist for price ideas on what the D-40 might be worth.
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Your best bet is - keep it for a spare body!
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Stay with Nikon, when you fully understand, you will be very grateful.
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$1500 dollars eh? That might just be enough to get a Nikon D7000 body WITH a lens. But if not, you luckily have your D40 and can use your lens for that camera on the D7000 if you so desire. I'd say go for Nikon if you've owned a Nikon. You're familiar with it's interface and how it functions, plus you already got some lenses.

You can probably sell your D40 for around 350-400 dollars depending on it's condition.

Well, this sounds like "more money than sense"…

the D40 is still a very acceptable camera… If you aren't getting what you want from the camera, perhaps it is your skill that is lacking…

OR you could blow your budget on another camera you can't get what you want from…

The D40 should be good for now. For good photos you need a good lens rather than a different camera body. So do a bit of research and depending on what type of photography you are most interested in, buy a new lens suitable for it. With a good lens you can shoot in low light, get better quality and greater depth of field.
Stay with Nikon. You can always upgrade your body later and use your lenses that you bought on that.
A Photographer once told me, for good Photos you need good glass! Heheh.

You're a beginner. A better/newer camera will not make you a better photographer. You have almost certainly not outgrown the fine D40 yet. I would suggest a better lens than the kit lens, although the kit lens is not a bad thing to start with.

Check what used D40s sell for on Ebay, or ask at KEH.com.