I would advise you to wait.
Buy an SSD + the an ebay optibay now but wait with buying the HDD.
Due to the Thailand flooding HDD prices are still very high. If you don't need the space right now. Wait another 2 months for prices to get back to what they used to be. Just use the current too small HDD in the main bay and install a SSD. Upgrade the HDD later.
I personally would always go for a 5400rpm drive if there is an SSD as a system drive planned. 7200rpm only if it is the system drive too.
I think it is too loud and for the stuff one usually stores on the secondary data drive random access is entirely unimportant for movies, music, images, other big files, backups
all that matters is squential read/write and in that respect the higher density 5400rpm drives are equally fast. Density makes up for the slower spinning and 7200rpm drives are always a lot more noisy and sometimes they add vibration which is really bad imo. 5400rpm usually have never any noticeable vibrations.
With an SSD definitely go Scorpio blue or Samsung M8 but wait if you can.
Unless you mean to use the HDD for a system drive, which I wouldn't advise. Would be a waste to buy the SSD in such a case.
Some Tests
http://www.storagereview.com/western_digital_scorpio_black_500gb_review_wd5000bekt
http://www.storagereview.com/western_digital_scorpio_blue_1tb_review_wd10jpvt
http://www.storagereview.com/seagate_momentus_750gb_review_st9750420as
And for 95-99% of all use cases of data drives the only metric that matters is squential read/write. Everything else won't result in any noticable difference.