<p>First... Take a look at the first page or so of the "Graduate School Admissions 101" thread.</p>
<p>The real, huge, difference between undergrad vs. grad admissions is that it is completely impossible to determine any given individual's "chances". At best it is possible to suggest someone is "not out of it" based on things like GPA and GRE scores.</p>
<p>As others have indicated, ECs are completely irrelevant. Prior research experience can be important, if only to show you have some idea of what the field involves. </p>
<p>GPA in major and relevant coursework will be important, but mainly as a disqualifier, that is, adcoms are looking for good grades (3.4+ for example) but also recognise that grades are only one indicator. As or more important is WHAT you took. In my field, Classics, everybody has 4 years of Latin and 3 or 4 or Greek. The applicant coming in with German too will stand out. </p>
<p>GRE scores (general and subject) are another "disqualifier", i.e. they won't get you in but can keep you out. Anything over 1200 V+Q will be fine. One thing a high GRE score WILL do is get you into university-wide fellowship competition. </p>
<p>Your LoRs will be very important. Academia is a very small world - in any given subject area, chances are you are no more than 1 or 2 steps from the top people world wide. So even if you are going to East Bumcrack College, chances are good your professors were drinking with their peers from Harvard at the last big conference. If you are going to a high end school you may even hear things from your advisor like "Oh, University of X? We can get you in there." (Yes, I actually heard that from my undergrad. chair.)</p>
<p>But the X-factor is that in any given year for any given school, there is no way to tell which professors will be taking new advisees or what their interests will be. What direction is the department taking? Is it growing or shrinking? Do they have a bunch of grad students who are way behind and are getting heat from the chancellors?</p>
<p>The whole process is opaque from the outside yet intensely personal. So you do your best, don't sweat the occasional B or C, get to know your professors, get involved with undergrad research or at least take classes with a large writing component in your field, and, well, place your bets.</p>