++ on the tripod
Depends how dark it is but if it will be beyond 30sec you will need a remote to hold the shutter open in bulb mode manually.
FWIW, I've rarely found I need to go beyond 30 secs in an urban situation. Even at night, there's usually enough light around to keep the exposure time down unless you're pushing to f14 or further.
I tend to use a high iso to take a faster shot to make sure the focus and framing is ok then shoot at iso100 for the proper shot.
Useful advice if it's going to be a very long exposure.
Might be a good idea to set long exposure noise reduction on too if you get too many hot pixels.
Bear in mind that if you do this, it will double the exposure time - the camera takes another 'black' shot of the same length as the original exposure to calculate where the hot pixels are.
In practise, most modern software (certainly Lightroom and I'm fairly sure recent releases of Canon DPP) maps them out automatically if you shoot RAW (I'd recommend it just for the ability to adjust the white balance reliably later on night shots). I rarely bother with the feature these days.
One other thing you have to watch out for, especially if you're taking pictures of floodlit bridges, is that you may either blow the well-illuminated bridge or leave the rest of the scene in gloom.
I don't know how the 350D performs, but I tended to find my old 300D would generate an appreciable amount of noise in the shadows on long exposures, even when the scene was properly exposed. This meant there was little leeway for lifting them up in PP. The 5D which replaced it is considerably better in this respect.
Bracketing your exposures (EV -2, -1, + 1 and +2) may help find a reasonable compromise or allow you to blend them into an HDR later.
P.S. Investigate using the mirror lock-up custom function on your camera.