That's up to you, there is no objective answer. Asking online you're going to talk to an abnormal distribution of people that are gear-dependent/enthusiasts; they tend to encourage upgrading. Which is fine, if you have money to spend and want to upgrade, then go for it.
However, it sounds to me more like you want to upgrade, than need. You mention how old your camera is, and how far it's traveled, but you don't mention any shortcomings. Is your camera limiting your photography somehow? Is it unreliable? Are there features on modern cameras that you need? I'd say that the "when", is when you answer yes to those.
The XSi is perfectly capable of making large prints; to an extent. I too took mine around the world, and put over 100,000 actuations and many many miles packed into an over-full backpack. It's still clicking. I upgraded out of want over need as well. My XSi made me enough money to pay for all my equipment and I had money to spare, so I eventually upgraded to full frame and made up a list of reasons why I needed it. I didn't, but I don't regret the upgrade.
I digress, but my point was stated. Personally I think that lenses are more important than cameras, but no doubt a modern camera would be a nice upgrade from an XSi. But you can answer which camera without answering what you'd like to improve in your XSi.
