usually older asus laptops have a recovery partition to restore a system (it's called f9 recovery). you can create a copy of the recovery files in usb stick, or dvd (multiple dvds), using the tool provided at asus support page for your laptop (it's called asus backtracker). you can use this to restore the system. alternatively, put in ssd, clone the current os and system partitions to it, put the hdd in hdd caddy and replace bd drive. i will suggest you keep the stock hdd intact, i.e. take it out of your computer, and don't do anything to it first. once your system running ssd is tested to run properly (the first thing to do is to check if your os is restored and is activated successfully), then you can do whatever you want to the original hdd. this is optional, but it's the best fail-safe way to do things, so that you can still use the laptop if something screwed up.
it's better to NOT use ssd with bd drive sata slot. for some reasons, in older asus laptops, the sata port connected to bd drive is much slower than the ones in hdd bay (in my old g55, the ODD uses sata1 (1.5Gb/s), while the hdd bay uses sata3 (6Gb/s)) so to get the best performance, use ssd in hdd bay, and hdd in hdd caddy (replacing bd drive).
edit: forgot to answer questions :/
1. with the processes (system recovery set, and cloning) your original windows activation state is usually preserved. this is always true if you do cloning, and the upgrade doesn't involve mobo.
2. ssd is much smaller, so yeah plan your migration. for cloning, usually it's enough to store os (without user data) and important system partitions. if you have the habit of keeping all files in c:, move them out and store them else where. imo it's better to always keep your data outside of your os partition.
all in all, it's good to make a full data backup just in case.
Ah, thanks for this suggestion! 1.5GB/s should more than suffice for the HDD right? Or does it bottleneck the HDD to a certain extent? Not like I have much of a choice here, but I'm curious to find out.