I use 1gb cards in my 300D. I get about 250 shots per card, in JPG mode at BEST/FINE resolution. NOTE: There is a number on the rear LCD that tells you the estimated number of pics you can fit on your memory card at all times.
I prefer using smaller cards in my camera, so that I can get about 1 day's worth of vacation pics on each card and don't have all of my eggs in one basket. I rarely fill a whole 1gb card even on a heavy shooting day in the parks.
Don't worry too much about loading the software for the camera on your computer. I don't even have mine loaded any more. Instead, invest $20-$30 in a memory card reader that will handle Compact Flash (CF) cards. This is how I get all of my pics off the cards; I never even connect the camera to my computer. Downloading pics with a USB card reader is much faster than downloading using the camera and USB cable.
The 55-250IS lens is physically about the same diameter as the kit lens, but about twice the length. It's not terribly heavy, either.
I recommend that you forget about setting the aperture for now and keep the camera in P-mode, which sets both shutter speed and aperture for you. This might help you with blurry pics, except when you try to shoot a moving target.
The LCD on the camera is not the greatest place to look at your pics. They look lousy no matter how much you enlarge them. Once you get your pics on the computer and view them with something like ACD-See or Photoshop Elements, or even the Windows Vista slideshow screensaver, they will look worlds better.
Having to compose your shot in the viewfinder rather than on a live-preview LCD is a good thing. It forces you to press the camera to yuor forehead, making it much more steady, and forces you to pay closer attention to the composition of your shot. You'll get used to it in short order, and once you do, you'll look on those who compose pics on the LCD screen with utter contempt - "Why don't you buy a REAL camera, with a viewfinder?!"

Well, maybe not, but you wil get used to the viewfinder. Besides, the viewfinder is easier to see in bright sunlight than any LCD.
I agree with BobQuincy that you should lock the focus point on the center. This way, you'll always be focused on whatever is in the center of the frame and not have to worry about the camera accidentally focusing on something that's off to the side.