Which Camera Brand Is Better?

I have a Canon Rebel XT right now but i want to get a Nikon D3000.is it worth it? Is Nikon Better?

No, neither are better BRANDS, but hte D3000 is quite an improvement over the Rebel XT. I'd push a bit further and buy the current model though, the D3100. If at all possible I'd push that even one step further and go with the D5100. Big improvements over the D3100. However the D3000 isn't BAD in any way.
It is a great, entry level camera designed for a very basic beginner. It has all of the abilities of your Rebel XT and more (much more.) It even has a module to it that will help you to understand the camera and get to manual if you aren't already there. Great learning camera, amazing family camera, great beginner SLR.

Both brands are high quality. It comes down to which model you like to handle better, and the lenses you plan to buy in the future.

As the other poster said, the Nikon D3000 is a big improvement over the Canon Rebel XT. On the Canon side of it there's also the T3i to look at.

Go to you camera shop, and handle the models by Canon and Nikon, go through their menu's and see which you find easier to navigate intuitively. In the lower end Canon's and Nikon's like the D3000 and the T3i, there's not too much difference in quality. Once you move into the higher end, Nikon takes the prize in low light performance, and Canon is stronger in the realm of DSLR video.

If you are just an enthusiast, keep it simple - pick the one you that simply feels better to you in regards to buttons, menus, and just overall ergonomics.