you had some pretty competitive and consistent scores to begin with, but with your gpa definitely retake. just study the same way, but do a little more.
for chem i definitely recommend destroyer and kaplan subject tests if you can get your hands on them. i found them to be really helpful, and similar to the actual thing. Destroyer is a little overkill in gchem but with both of them i went from a 16 > 22. Use Cliffs AP Bio for biology or Campbell's textbook. your PAT is good, but it wouldn't hurt going for a 22 this time, 20 is definitely nice, but a few months of tweaking and you could pull it off.
good luck.