Not too long ago I bought a Canon Digital Rebel XT. It was my first SLR camera and I've been really pleased with what I've managed to do with it so far. I have the basic 18-55mm lens that came in the box and a very basic 75-300mm lens that I bought later on. I was wondering what to do I have to do (hardware wise) to take my photography to the next level? I was wondering about maybe experimenting with some filters. Heard polarizers were good. Also I was thinking about buying an inexpesinve ( <$150) Macro lens to get some interesting close up shots. So if anybody has any ideas or thoughts, they would be greatly appreciated

The very basic (and inexpensive) filters to have are:
a Circular Polarizer and
a Graduated Neutral Density filter.

Also spending money in a good tripod and a TTL flash are always a good investment.
Canon Digital Rebel is a good camera (in europe is sold as Canon 350D) and the lens kit is not bad at all.

Personally I have no experience with it, as all my equipment is Nikon, but one friend of mine have it and I saw spectacular photos made with it.

Don't worry too much with hardware and take photos. Some of the best fotos has been taken with non-pro cameras !