what is the difference between the 08 civic SI and the 07??

l dont see anything different, do you.
same body style, l dont think they had any changes.

For the 2008 model year, the Civic Si received minor tweaks that include even darker wheels than the 2007 model and a new shift boot with red stitching. Mechanical changes included a new tire pressure monitor system, a new rear upper arm that decreases the amount of rear camber, and a lowered spring rate for the coupe to match the sedan

ivtecpakiness is right, the most notable change is the addition of the Tire Pressure Monitor because is mandatory for every 2008 car.

But wait a bit longer you can get a new 08 in great price since the 09 models are around the corner and it comes with a few body changes and new colors.