I don't think these forums are the place for this sort of question, it's not really a techie type forum. I don't think it has that kind of expertise.
If your provider is Orange, then you're stuck with Android 2.1 as there aren't any upgrades available with Orange's offering. However, some flush out the Orange modified version of Android and install a more adaptable version. What do I mean by adaptable? Well, on your Orange version of Android, you can use any Android software compatible with ROM versions up to 2.1. If you flush out the Orange version and install the common Android ROM you can install app's that will run on ROM's up to the current 2.3 version of Android. Orange's version of Android is not upgradeable, but the common version of Android is.
You may not have Orange, but if your phone and version of Android is branded, the same principle may apply. With the common version of Android, the ROM manager will assist with the process of upgrading your ROM from 1.5 to 2.1, or 2.1 to 2.3, etc.
I don't know your level of knowledge with these matters, so please forgive me if I've over-simplified this explanation.
If there are any on these forums familiar with the techie aspects of Android and the OP's question, please correct me if I've said anything wrong.
