Many on this forum and elsewhere have gotten excellent results with the unmodified 300D and a Hoya R72 as woodsac said. The vast majority of people removing the built in IR cut off filter are doing so to use the camera for astrophotograghy, shooting very dim subjects that emit a fair percentage of thier light in near IR. The only downside to leaving the filter in is the long exposure times, you won't be shooting any action. Average exposures are around 30secs so a tripod is a must.
In addition to the filter you may also want to consider getting a 50mm prime or other suitable lens if you don't already have one. Many lenses produce a "hotspot" in the center of the frame when shooting in IR. It seems some Canon lenes are not suitable and others are just fine, some say it has to do with the anti-reflective coatings used.
Here is a list I found second hand, Im not sure who originally compiled it, hope it helps.
Good
Canon EF 17-40mm L f/?
Canon EF 24-70mm f/2.8 L
Canon EF 28-135mm/3.5-5.6 IS
Canon EF 28mm f/2.8
Canon EF 35mm f/2
Canon EF 50mm f/1.8 MKI
Canon EF 50mm f/1.8 MKII
Canon EF 70-200mm f/4L
Canon EF 75-300mm/4.0-5.6 IS
Canon EF 135mm f/2L
Canon EF 100-400mm4.0-5.6 IS L
Canon EF 100mm f/2.8 macro
Bad
Canon EF 16-35mm f2.8 L
Canon EF 20mm f/2.8
Canon EF 24-85mm f/?
Canon EF 28-70mm f/2.8L
Canon EF 28-105mm f/?
Canon EF 35-80/4.0-5.6
Canon EF 50mm/f1.4
Canon EF 50mm/f2.5 Macro
Canon EF 70-200mm f/2.8L IS
Canon EF 85mm f/1.8
Tamron 70-300mm f3.x-xxx Macro