First of all, don't panic! Your plan sounds fine for MSTP prep. As immuno has said, you should focus most on your undergrad research. You can also do research during the school year, so that you can work on one big project rather than dividing your time into summers and not really accomplishing anything (unless you happen into a lab at a fortuitous time).
Regarding tests: I've taken the MCAT and GRE, though not a subject test. The GRE general test is the easiest of the bunch. Unless you need a perfect score all you will really need to do is to take some practice tests from ets (they are free) to get a feel for the computer format. You can also take it pretty much anytime. The MCAT requires some prep, as you are going to want to score well (34 and above is a good goal), though many people do get into MSTPs with less. From what I have heard of GRE subject tests, they are extremely difficult, so you may want to save that one for last, after you have studied for and taken the easier MCAT bio section.
Finally, if this all seems like too much to do while an undergrad, I second immuno and say: take time off after undergrad! It's not a race. If you can focus on research as an undergrad (15hrs+ a week during school, full time during summer) and make great grades (especially in upper div) you will be in a strong position for MSTPs. Taking some time to nail tests after that won't make your chances worse, and you could do something really great with that year, like travel, play in a band, learn a new language, get published with your lab, etc etc.
If you don't do residency, you can't practice. Some people who only want to do research go this way, though it does limit your options. You'll probably want/need a postdoc if you are going into research. Yeah, that's several more years (after the mstp 7-8) between res and postdoc.
Whatever you do, good luck!
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(I hate apt hunting...)
