You should not expect a big difference if you change your camera to a Xs/1000D, XSi/450D, T1i/500D, T2i/550D, or even if you want to stay with Nikon and to get a D3000 or a D5000.
Unless you just want to get a half-stop better high ISO or video capabilities, you must at least upgrade to the Nikon D90... or you may go to another level and get a Canon 50D. If you want to go semi-pro, get a Nikon D300 or a Canon 7D, or the full-frame bodies Nikon D700 and Canon 5D Mark II.
I really recommend you staying with Nikon, since you already know how its system works, and to get new lenses like 35mm f/1.8 DX AF-S, 50mm f/1.4 AF-S, 70-300mm AF-S, and also a external flash like the SB600... in the future you may upgrade to a full-frame body with a 24-70mm f/2.8 and a 70-200mm f/2.8, and buy a SB900 to be able to use the SB600 as wireless slave flash.
Switching to a Canon system is not a great deal.... and changing from Nikon to Canon is not worth too... both systems are great, both have a great collection of lenses and good wireless flash systems. Just stay with what you already have.
Many pro's switched from Nikon to Canon when Canon introduced faster AF lenses, but a lot of people switched back to Nikon when some Canon top pro cameras began to show defects in their AF systems. But both systems are very realiable now.
Canon is most used for wildlife and sports, while Nikon for events and weddings. But it's applicable only to the professional applications.
Usually the Canon entry-level dSLR have more features, while the Nikon entry-level dSLR have easier automatic shooting modes due to a better metering system. But it seems Canon is implementing better metering systems in the newer cameras like the 7D and the 550D/T2i.
I choosed a Canon Xs/1000D over the Nikon D3000 because it have a better high ISO, liveview, better RAW development software, more dedicated buttons and more information in the viewfinder. But I would choose a Nikon D5000 over a Canon T1i/500D if I was searching for a camera in that price range.