Your question is a bit like asking – I wanna go fast what car should I buy?
How much do you want/have to spend? What type of footage do you edit – SD, HD, 2K, 4K? What codec do you use most often – RAW, ProRes, h.264, DNxHD, etc.? Do you prefer a Mac or Windows? Do you want to run other apps besides CS4/CS5? If so, which ones (Avid, FCPX, Smoke, Resolve, etc)? What version of CS4/CS5 do you have – Mac or Windows? I believe there’s still a small fee (shipping costs…maybe more) to cross grade from one system to the other.
All that to say, you’ll need to be more specific about what you want/need if anybody here is gonna be able to help.
My $.02
Ryan Holmes
http://www.ryanholmes.me
vimeo.com/ryanholmes