You should go ahead and do what you first propose of the SSD in the optical and the HDD in the main.
There are two things to consider with speed, data rate and latency. Data rate is how fast the data can transfer from point A to point B. Something to keep in mind though is HDD are not that fast and may not use the full 3 Gigabits it is given. A WD green drive is made to save power and reduce heat. This means it spins slower, so it can't pick up the data as fast. As a result you are not at transfer link saturation.
The SSD does not need to spin and and only access one part of a file at a time, it has the entire drive accessible. As a result it can saturate the line.
Latency is also important, it takes time for the HDD to get to the files that are needed, for the SSD the files are already there. As a result, during boot the computer is not waiting for the next file to come around. It can go file after file after file. Reduced latency means faster speeds.
Already your first proposed setup is wining, but while doing some research, I found answered question: What is an SSD drive's negotiated link speed?. The optical bay is capable of 3 gigabits per second, however, the negotiated speed is 1.5 because the optical drive is slow and doesn't need the 3.
The optical drive negotiated for a lower speed but can handle faster on the bus. If you already have the drive one thing you can do just test it out yourself, try one configuration then another. There is a free program called Black Magic disk speed test (itunes.apple.com/us/app/blackmagic-disk-speed-test/id425264550) that measures your hard drive speed.
re 1: hard to say - if you can spare the Money you should go for the 850 bc. of a (probably) better Garbage Collection (see #3), else go for the 840.
re 2: The best tutorial for replacing the optical drive with a harddrive (or SSD) can be found at iFixit.com, f.e. this one. You should search for the Tutorial for your model, the link i posted is probably not for your model.
re 3: There is a hot discussion going on if Trim should be enabled or not and if that is a bad thing. Fact is, Apple has Trim Support enabled only on its own OEM SSDs and disabled it for third party drive. There are Tools to enable TRIM Support, but it could be a not necessary bc. of a better Garbage Collection within the SSD Firmware (see this article and this Article). For general Infos i recommend this Article from Samsung
BTW, to create a homemade Fusion Drive you need to use the 'Core Storage Volume' Command. You can look it up with
man diskutil
look at the 'coreStorage' section. There are some tutorials for creating a homemade Fusion Drive, but it's a very painful process.
Best Answer
I guess you figured this out a year ago now -
but you need a PATA -> SATA power adapter [couple of quid/bucks/shekels from Maplin's/Radio Shack etc] & run it from one of the old superdrive power-supplies you'll find ready & waiting in that bay.