The Olympic-Bar curl is the single best movement for building Bicep mass, while dumbell curls put a better 'peak' onto your arm. I also do Hammer curls, as they hit the outer bi's and upper-forearms hard. Cable curls are beyond useless, and Preacher curls are only effective if you lower your arms past 90-degrees, which very few people actually do. Personally, I fell that lower reps with heavier weights are more effective at building bicep strength. That said, my bicep routine is thus:
Alternating Dumbell Curl:
12 x 50(warmup set)
12 x 60
Olympic bar curl:
8 x 110 (warmup set)
6 x 135
45-degree Hammer Curl:
16 x 35 (warmup set)
12 x 40
Try this one twice weekly for 4 weeks. It may not look like much, but if done with minimal rest between movements, using the heaviest weight possible, it becomes very difficult to complete. Perform 2 forced reps on all movements after each working set. Correctly done, you may even be able to curl bodyweght after time, which is currently my short-term goal (I'm almost there)...
By the way, due to the fact that your nom de guerre is 'Welsh Power House', I found myself wondering where you live. You see, about 99% of my family lives in Wales, although I myself live abroad, unfortuantly. Another question: do you know how to speak Welsh (Cymraeg)? In my youth, I learned it for novelty value...
Regards,
Deathspell Omega