I have 2 1TB Blacks, one is 32mb the other 64mb cache. Would I have any long term probs? Ive had them raid'd before(on my EVGA x58), but only a short time, before I relized they were different cache sizes.
 
THX
 
 

I can't see why, but you speed might not be the best 

I have two also, one bought Jan and the other is years older, they differ in performance enough that I would not - I would get a 2nd new model. There is more than 4% variation.
I'd think a mirror you might be able to get away.
And now WD has a new 10K VR in 1TB $290 / 200MB/sec models and smaller 2 x 500GB @ $200 each. But close to 400MB/sec also.

 
General rule is when you are building a raid, all drives must be identical. That includes cache sizes. 
 
You can still build it but it really depends on what type of raid you plan on setting up. If you are doing a raid1 then you will probably be ok, but if you go raid0 then long term problems can be random file corruptions, to full on failure of the array due to the controller glitching from overloading the cache on the smaller drive.