When you add RAM and a battery-backed or flash-backed cache unit to a Smart Array P410 controller, you will gain write performance and unlock a few features. You'll be able to perform an array transformation (e.g. convert from RAID 1 to RAID 5) online. And yes, you're correct. RAID 5 is not available on Smart Array P410 controllers that do not have a cache and battery combination. Other than that, the controller and the RAID array data is unchanged.
My usual warnings apply: You should perform this work using the HP Array Configuration Utility, either offline from the HP SmartStart CD, or with the HP ACU utility for your operating system.
Please update the firmware of your controller.
Each revision of the firmware adds functionality and fixes bugs. It's usually a good idea to do this.
Also, while I know your goal is RAID 5, have you considered RAID 1+0? If speed is more important and capacity isn't an issue, I would definitely choose RAID 1+0 over RAID 5 in that setup. See the canonical RAID discussion for more detail.