Injen is probably the best bang for your buck. They claim like 10hp, but the difference to the wheels will not be noticable. It does have a great sound though.
A few others:
Comptech Icebox
Fujita
Weapon R (not sure if they make one for the si yet, but I hear they're pretty good)
K&N
...and as you mentioned AEM
Hope this helps.
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'07 Alabaster Silver Metallic Civic Si
Mods
Injen CAI
Injen SES Cat-Back
DC Race Header
