How many MP is a 1000D? 10MP? You'll be able to get around 250 Raw shots on a 4GB card. I have a 7D and use 8GB cards and i can get 270-300 Raw shots but the file sizes are around 20-30MB.
I always shoot RAW. Much easier to manipulate the file. You can do it with a JPEG but it will not be as clean and it won't take much for the file to basically "crumble".
A stop is 1 stop. E.g. F5.6 to F8 is one f stop. 1/125th to 1/250th is 1 stop etc... Google it, if you want the full stop calculations, i don't want to type them all up, will take too long lol.
RAW's advantages over JPEG is easily the manipulation. You would be surprised how much you can recover in a RAW. Although blowing out a white shirt and pulling it back 3 stops might be an extreme as it will start to lose it's dynamic range. If that is the case, i suggest you work on your exposures.
If you shoot a JPEG it will already (In-camera) adjust the sharpness, saturation, contrast etc... to whatever you have set in the camera. Whereas the RAW PREVIEW will show exactly the same thing, but once loaded into camera raw in photoshop, these will be stripped and the image will be "flat" until you adjust it to how you want.
Shoot an image 2 stops over-exposed/under-exposed as a JPEG and shoot the same thing as a RAW and you will see the difference. Try recovering them both and you'll see the difference.
Most sport shooters shoot in JPG at events so that the "card runners" can collect their cards and upload straight away to the editors etc.. 95% of pro's/most photographers will shoot in RAW. Much less destructive than JPEG editing.
IF you google RAW vs JPEG i'm sure there are over a million answers why you should shoot RAW