I would get the biggest ones you can afford for the most gains. You will need cam shaft seals and I would also upgrade to adj. cam gears (I highly recommend them) at the same time. That way you don't have to redo the timing twice if you decide later on down the road. Also you will need to get them dyno tuned in order to feel the benefits of the cam gears. I had the stock cams dyno tuned and I gain power everywhere. Really livened the motor up. I have yet to get the cams I have now dyno tuned but I imagine I will get the same results. I'm currently running a Ford Racing stg. 2 intake cam and a Crower stg.1 exhaust cam. The only reason why I'm running two different kinds of cams is because I bought one cam at a time used due to financial reasons. They popped up on the BST forum so I scooped them up.
Here is the How to for the cams HERE
Here is the cam gear how to right HERE
I want to say that Comp Cams are the go to cams that one of our tuners highly recommends. They have great drivability and fantastic top end power.
Oh and also if you haven't gotten a tune from a tuner I highly recommend that as well. I gained so much low end power from the tune and not to mention the huge increase in driveability that came with it.