Well there's the Linux+, RHCT and RHCE certifications which seem to be the most often talked about and well known. From what I can tell there arnt to many jobs that call for the Linux+. It may help you out if your trying to get a job in a cross environment but it's not likely to as useful in the aquisition of a job as either of the RH's would be. At least from what I can tell. I personally am aiming high, and I know it, and going with the RHCE. Something I found out was that an RHCT can be aquired even if you dont pass the full RHCE. It's really going to come down to how hard you want to push your self and how much you want to learn to get to what job level.