Regardless of whether you have a Rebel, or a 1Ds, there are certain things every Canon owner should have.
Most of the consumer DSLRs are very good, a 400D or a 450D is a great choice for starting to shoot stock photography. There is no need to upgrade to the professional models too quick.
You are much better spending money on the glass – get some great lenses.
If you are pushed for cash, the first thing you should buy is a great portrait lens: Canon 50mm f1.8II. You can buy these for around $100 and they are very very good.
Next you will need a flash. It is well worthwhile spending the money and getting a 580EX. They just save so much work. If you get a cable you can use them off the camera as well. Some strobist sites even make suggestions for the use of the flash using umbrellas etc..
Next, an L lens. You must have an L lens. Before you even think of upgrading your camera. The Canon EF 24-105mm f/4 L IS USM Lens is the best all round lens in the stable. It is something you will have for life. From the wide angle of 24 to full zoom it is sharp and fast. It has image stabilisation. It will be great for landscapes, weddings, indoor product shots. You name it. If you do not buy this lens, it is because you are more focussed on portraits, in which case you will get the Canon EF 85mm f/1.2 L II USM Lens. This would be your next buy anyway.
Finally, September is rumoured to be the release date for the new Canon 5D. That means the old 5D will sell for even less than it is now. A full sensor size camera with all the bells and whistles, and it will come in under $1500.