If you stear clear of Apollo1230's (due to incompability with som A1200 motherboards), GVP 1230's (due to their use of special mems that are impossible to find at a fair price) and any 1230 card that doesn't support at least 64Mb mem you'll probably do just fine.
Find out if you need SCSI option as some doesn't have this and some are extremly hard to get.
Next thing to considder is FPU, but to be honest most of us don't need it so unless you know you're going to use alot of math intensive programs just ignore it and see it as a bonus if it's included. Most software doesn't benefit from the FPU and even those that do doesn't have much to gain from a 68882 FPU.
Remember that even a "slow" 28Mhz 030 is still about 6 times the speed of a stock A1200 and it will deffinitly be noticed. The 50Mhz do give even more boost but it's not nearly as much as you might expect. What I'm getting at here is that you might not have to look blindly on a 50Mhz card... if you need some extra speed 28 or 33Mhz will most deffinitly do and even if you need some more speed a 40Mhz card will do almost as good as the topend.
Remember this is my oppinion.. others might have othre oppinions about the issue.
