Well, our school has us take the other exam as a practice near the end of 2nd year. Obviously, you would study for a while between that and the real Step I, so the correlation of scores is difficult. However, the questions are very similar in difficulty and in content. The real Step I is much longer and requries more stamina. At our school, most people went up about 20 points from the CBSE to the USMLE Step I after about 3-4 weeks of preparation.

My school also has us take the test for practice at the end of 3rd year, and again during 4th year as practice for step 2. I would say it is very similar, but shorter. Most people take the practice test cold, so almost everyone's score goes up quite a bit. I scored a 205 for the Step I practice with no prep, and my score went up about 50 points for the real test after a month or so of studying. For Step 2, I scored a 215 on the practice and my score went up about 30 points with one week of study. Some friends of mine scored around 170-180 on the practice, and went up to about 220 for the real thing, so you can raise your score substantially with a few weeks of study.

Raven,
I sent you a PM.

Now that the Step 1 scores have returned, I can comment on how those NBME basic science tests correlated for me. I got a 235 on the one issued by my school at the end of 2nd year before starting my Step 1 preparation. After about 3.5 weeks of hard core study, I took the Form 2 test from the NBME website (costs $45) and scored 253. Seven days of study later I took the real deal. I just received my score and was very happy to see that it had gone up. So, the NBME tests do seem to be good predictors of Step 1 performance and seemed to chart my studying progress pretty well. I recommend them.

I took the Form 1 test about 2 weeks before the USMLE and got a 240. I took Form 2 four days before the real test, thought it was more difficult, but got a 245. My actual score was a little higher than that, so I'd agree with BoneFlute in that these were good predictors, and I felt that they were quite accurate in representing the types of questions found on the real exam. I recommend them as well. If I could do it all over again, I'd schedule these forms like BoneFlute did to give yourself more time to study between them, and after the second form before the actual test.