MOD EDIT

Mats, head to http://www.everymac.com/systems/appl...drive-ssd.html it helps with identifying your Mac and links to the relevant Apple guides on how to replace your disk It also states the earlier models 1.5gbps limit was removed since with a software update but people above are correct in that limit is irrelevant.

My guess is you have a MacbookPro8,1 http://www.everymac.com/ultimate-mac...=MacBookPro8,1 or perhaps a 8,2 not much of a difference. You will be able to, without much effort (remember, even Apple has a guide to do this), replace the HDD with an SSD. Here's https://www.ifixit.com/Guide/MacBook...placement/5119 the ifixit guide. Note the hardest step there is disconnecting the battery which might not be necessary (it's good to protect the system from mishaps of course but perhaps ifixit wants to sell you a spludger -- who knows ) but for example the instructables http://www.instructables.com/id/Inst...Book/?ALLSTEPS guide skips it. I am taking no responsibility of what you are doing with your MacBook of course

Also you can put the existing or a new HDD (Samsung Spinpoint M9T comes in 1.5TB and 2TB sizes, for example) into the optical bay as well if you so want. This requires a simple, cheap adapter (even OWC, the king of fleecing Mac people only asks $35 for this adapter, on eBay you can find many for $6 shipped).