IR photography is completly possible with an XSi.
However, in front of your CMOS sensor, there is an IR-cutter, a filter that prevents IR light from hitting your sensor.
To capture IR photographs, you can do one of two things:
1. You can buy an IR filter- a filter that filters out all VISIBLE wavelengths of light and lets in only IR light. However, since you have an IR-cutter in front of your sensor, only a small amount of IR light will reach your sensor and hence, you will need to use a longer shutter speed.
2. You can buy an IR filter and then remove the IR-cutter in front of your sensor either by yourself or by a third-party. This will allow you to capture IR photographs hand-held. However, this effectively removes most functionality of your XSi as a regular, visible-wavelength light camera. While your camera will still be able to capture regular photos, they won't look right due to IR interference.