I just read that the Si uses 97-01 Prelude rotors.
http://www.8thcivic.com/forums/showthread.php?t=62478
I know the front rotors and I believe the front and rear calipers are the same as the Si's except the left and rights are swapped for the front to get the bleeder in the right position (RSX brakes behind the hub, Civic brakes in front of the hub).
Also, here are some of my findings from Brembo's site:
25550 - Acura 02-06 RSX (Rear) Diameter 259.7mm, thickness 9.0mm, min thickness 8.0mm, height 48.3mm
25550 - Acura 97-01 ITR (Rear) Diameter 259.7mm, thickness 9.0mm, min thickness 8.0mm, height 48.3mm
25550 - Honda 98-02 Accord V6 (Rear) Diameter 259.7mm, thickness 9.0mm, min thickness 8.0mm, height 48.3mm
25686 - Honda 97-01 Prelude (Rear) Diameter 259.7mm, thickness 9.0mm, min thickness 8.0mm, height 42.2mm
If the Prelude rotors are the same as the Civic's, they will not work in an RSX. And RSX rotors MIGHT work with spacers between the hub and rotor, but then your wheels would stick out 6.1mm further per side.
The reason I'm researching is because I'm having problems buying ATE Premium One rotors for my 93 Del Sol with 06 Civic Si engine/tranny swap. I have the Integra Type R suspension and brake conversion but the store I'm buying from says they don't have ITR rear rotors but has RSX rear rotors and so on.