Had the same decision to make around two years ago, and switched to the XTi/400d.
The 300d was a good camera, but the XTi had several advantages -- stored images more quickly, much faster at USB tethered operations, shorter shutter blackout, no four-frame continuous buffer limit, more durable shutter mechanism, less high ISO noise, wider in-camera saturation adjustment range, smaller body, better E-TTL flash performance. The XTi has worked so well that there are two in the bag now (one is a refurbished body that was available for US $340). The only major added expense to the switch to the XTi was getting new batteries.
A 300d was a decent quick replacement when the D30 broke, but its operational limits made a switch to the XTi a good decision when XTi prices dropped in 2007. One of the useful tricks with this equipment is to stay an affordable generation or two behind the state of the art.