Both CPUs are based on the Sandy Bridge architecture, but the Core i5 is a substantially faster chip, runs with four (two physical, two virtual) cores instead of just two, scales to 2.5 GHz/3.1 GHz versus 1.9 GHz, has 3 MB cache instead of 2 MB while running at the same TDP of 35 watts.
What makes this processor an interesting choice is the fact that it has a rather high tray price of $225 compared to the $86 tray price of the Celeron B840, especially in the view of the $499 MSRP the Chromebox is selling for. It seems that Intel may be pushing its remaining Sandy Bridge stock out rather quickly and may be granting heavy discounts, which may explain the hefty CPU upgrade. The Core i5-2450M in the Chromebox is, by the way, very similar to the CPU Apple offers in its base $599 Mac mini setup, with the exception that Apple claims that its i5 chip has 4 MB cache, which is not officially offered by Intel for the i5 series.
Besides the i5-2450M CPU, the new Chromebox XE300M22-A01US comes with 4 GB RAM, a 16 GB flash of storage, six USB 2.0 ports, WiFi, Bluetooth, Ethernet, as well as DisplayPort and DVI interfaces. It appears that the price of the new Chromebox has already dropped from its $500 price tag and is currently offered from $397.