Stefano, there are two types of digital SLR cameras you might consider: true single lens reflex cameras that have removable lenses, and 'SLR-like' cameras that look like an SLR, but have a non-removable lens.
The 'SLR-like' cameras are considerably cheaper, but have some disadvantages. They have an electronic viewfinder, rather than an optical one; electronic viewfinders don't render fine detail as well, and can be hard to focus manually. The don't have a removable lens, which greatly limits their flexibility; and they tend to have smaller sensors, which translates into higher noise levels at higher sensitivity ratings. However, they are undeniably cheap. One good example in this class is the Panasonic Lumix DMC FZ8. The camera features a superb 12X lens, has a 7.38 megapixel sensor and can be purchased for as little as $283.00.
In the true SLR range, I would suggest that you consider either the Nikon D40X or the Canon EOS 400D /Rebel XTi. Both cameras feature a 10 megapixel sensor, which will provide you with resolution sufficient to make 20 X 24 inch - or larger - prints. Both of these entry level SLR cameras are going to stretch the upper limits of your budget envelope; but they both represent excellent value, and will do everything that you seek. In both cases, the body sells for around $700, and the lens is extra. The general 'word on the street' is that the inexpensive kit lens supplied with the Canon is actually a very poor lens; you will be much better served to purchase a better canon lens, even though this will elevate you above $750. The Nikon kit lens is better than the Canon, but still is not a superb lens; again, you might want to consider purchasing a better lens.
The cameras run neck and neck, and both have many fans. I find that the Canon is too small to hold comfortably; the ergonomics on the Nikon seem better. I also find that, for my needs, Nikon lenses are superior. However, the Canon is an excellent instrument, and Canon has served generations of photographers very well.
The camera industry is going through a metamorphosis. Konica merged with Minolta, and then Minolta got out of the camera business altogether and sold their imaging technology to Sony. Pentax, long revered as the manufacturer of the legendary Spotmatic film SLR, recently called off a planned merger with Hoya (a filter manufacturer). Bronica called it quits, and the word is Mamiya is on the ropes. For this reason, I would caution you against buying a camera from a lesser known name; you might well find that three years from now, your camera company has disappeared. I think Canon and Nikon will be around for a while, but I'm not sure of anyone else.
I would also caution you against buying a lens other than an original equipment manufacturers lens. That is, put a Canon lens on a Canon camera, and a Nikon lens on a Nikon camera. Don't be tempted to put a cheaper, 3rd party lens from another manufacturer on a camera body. The reason for this is that camera manufacturers regularly make system updates available; and frequently, those system updates can cause something like the autofocus or autoexposure on the lens to stop working, due to incompatibilities between the camera and the 3rd party chipset in the lens. Tamron has had a number of problems in this regard.
Don't be too influenced by 'megapixel wars'. Realistically, anything of 6 megapixels or more is going to produce good 16 X 20 prints, which is larger than most images ever get printed. And to double the resolution of a 6 megapixel sensor, you would need to find a 24 megapixel sensor! This is larger than any commonly available sensor; even the high end Canons, in the 5 grand price range, max out around 17 megapixels.
Of more interest is the physical sensor size. Smaller sensors need more electronic amplification, which turns into more electronic 'noise' in the background image. This can be minimized with aftermarket image processing programs such as Noise Ninja, but you're better off avoiding that road. Bigger sensors = better images.
I hope this helps you find a camera that suits your purposes. Good luck, and keep on clicking! Best wishes, Charlie Worton