Crypt is right -- you get evaluations for every rotation you finish as a resident. When you apply to fellowship, you submit Step I and II scores (III if you've done that, too), med school transcripts, LoR, a resume, copies of publications, Chairman's letter (IM chairman), personal statement, some even wanted Med School Dean's letter (keep all those addresses and phone#s handy!), a slew of other stuff I don't remember off hand -- PM or email me if you want the whole list, and I'll look it up. In my opinion, what makes your application stand out is:
1. Excellent evaluations as reflected by your Chairman's letter and LoR, and if you come from a top IM program, that means more
2. Step scores (but importance varies from program to program, I think)
3. Med school stuff
4. If you have research, this augments but I do not think supercedes the others. I had no GI research, truth be told, but started a small project with some GI faculty while I was applying.
Letters from folks at a top or well regarded IM program open the door -- this cannot be emphasized enough. Step scores and med school stuff solidify the case. When you have the interview, seal the deal -- really shine and look like a sociable, competent but not arrogant person that these interviewers wouldn't mind spending three or four years with training you.
Bottom line, get into the best IM program you can get into, and work your tail off. Your best chances are where you went to med school and where you did residency.
As always, email, PM, or post questions for me. Always glad to help