Why do you think the fan will be running at high speed if you add in a SSD drive?

Generally a SSD runs much cooler than a hard drive. The only rub here is if your constantly writing to them as the write cycle creates heat.

So the trick here is not to be constantly writing to the SSD to kept the heat down. But, you also want to do this to lengthen the lifespan of the SSD as well, as each write cycle wears the SSD.

So how you use the SSD is the trick. If you set it up as the boot drive and the location for all of your applications and use your HD to hold your data files (i.e. word & excel docs, music, pics, movies, etc...) then your not writing to the SSD heavily.

To lessen the writes even more see if you can move temp and cache files to the HD. You'll need to go into the application settings to alter them (if you can) in some cases you may need to tweak the application code directly (talk to the applications support staff they maybe able to guide you to where the settings are). But, before you go this deep make sure you have backups!

Most Mac apps have not been tuned to run on SSD's effectively yet. Even with the intro of the MacBook Air Apps have not really changed. Some apps now leverage RAM to hold temp and cache files and write them back when you close the app. The MacBook Air is still a different animal than what you have and Apple is just getting into the idea of a split storage system.

Your just ahead of the curve hang in there!