I just recently had to upgrade my 350D to a 30D (350D went for a swim in the Irish Sea) and I would never look back...
Spot metering, RGB levels report, larger screen and better form just make for a really great DSLR, I would have gone all the way to a 5D but I couldn't just go to that level of expense yet.
If you want to stick with the 350D why not go down the router of replacing some of your lenses for L series, this is where a hell of a lot of improvement can be made and also the L series lenses will fit on the 5D if you are looking to that option in the future.