My mini-background: undergrad math+biomedical engineering+premed; took MCAT a couple years ago and did very well (aced all but the verbal).
I'd hate to take ALL of one science contiguously, since then it's harder to reap its rewards on the other courses, but...I'd take Chm + Bio first year, then Phy + Orgo 2nd year (reasoning below).
But if you can split up each science into 2 semesters, I think you'll be better off...
First, note that many schools offer some of the 1st sem. courses only in the fall, and 2nd sem. courses in the spring. So if you're stuck on doing 2 courses a year, you may be limiting your options, depending on your school.
Phys. -- not really *needed* for any of the others, since where it comes up in general- and orgo- chem, it's explained, but one semester would help a bit for chem I or either orgo I or II, e.g., to understand various energy forms + their formulae, orbitals, photovoltaic cells... (if you were to take quantum mechanics, though, Orgo I would be CAKE!
Bio -- take the micro/cell half before any orgo (systems bio (II) doesn't help w/ orgo).
Orgo -- taking one bio and one chem will help IMMENSELY, and one phy will help some, and some of these may be prerequisites at your school. Orgo is sometimes hard to "picture" or to understand the big picture (molecular orbitals + energy levels, stereochem., relevance of the reaction groups), and these intro courses will help put things into perspective.
My recommended order, for maximum prep:
Phy I < Chm I (minor help)
Bio I < Orgo I (loads of help)
Chm I < Orgo I (loads of help)
I'd leave Orgo II for last, or near to last, if scheduling permits. Remaining 2nd sem. courses take in any order.
Hope this helps.
-pitman
