Looking to beef up my iMac and I would like to make it faster. Any tips or advise I have the 27" core i5 2.7GHZ
Beyond software fine tuning (which was in another post here and excellent) you can consider your hardware.
1) Increasing RAM may help quite a bit with some applications. If you have 4 gigs go to at least 8 gigs but 16 is a pretty good investment as long as you buy the correct RAM from a thirty party vendor.
2) Consider SSD drives. OWC along with others have some very good SSD drives that would show a vast improvement in performance. OWC also has a program/service to do the install as iMacs (unless you are very good at this) are not easy to open and replace drives.
Of the latter - I have put into a Mac Mini (server) two SSD drives and tested two scenarios - Single drive and Striped together. Both scored extremely well and far faster than the two 7200rpm drives raided previously in this server.
If you opt to RAID your drives, just be keenly aware that if one fails, you loose it all (in striping). I do backups daily which is a good practice for anyone.
If your iMac has Thunderbolt you can explore using external SSD or regular drives (raided) in external enclosures that would also show improvement.
Before doing any serious hardware changes best to read around the forums and the Internet for solid data to know what you are getting into. SSD drives are not cheap considering the lesser volumes. I opted for two 240 OWC drives striped together to give me a respectable volume. I use typical applications and graphic programs such as Photoshop. Your computer should be faster than mine and has superior graphics so again SSD should be a huge advantage along with increased RAM.