Back to the basic, the key features are screen resolution, speed of processor and version of operating system.
As an internet user, it is a habit for us to access it through the mobile. The higher the resolution, the more you can see and the less you need to scroll. We do not only need to read streams of text but also need to look up tabular information.
Speed of processor determines the rendering of web pages. However, I am wondering whether it is necessary to have dual core or tri-dual architecture because we don't expect to perform multiple tasks in large scale unless you are addicted to mobile action games. You may find that those phones with dual core have exceptionally high price.
There are some significant improvement in the latest version of operating system. Stick to the Froyo (2.2) or later versions. If you are price sensitive and you don't mind the speed, you may prefer Sony Ericsson Xperia X10 Mini Pro.
Here is my data: