Switching from DirecTv DSL to EarthLink DSL will have a negligable impact on your overall speed. Both companies would use the same exact line from your local CO to your house, as put in by your local telephone company...
Who is your "vendor", or local phone company? Some have minimums that they will ensure that your line is working with, others do not. Some areas also are technically not possible to get advertised dsl speeds, because of the backbone of the phone system to your house... Some parts of the Ameritech PSTN is a perfect example of this.
If your only getting 75kbps for upstream, yes that is better then dialup but NOT BY MUCH and is most likely the cause of your problems. You should not be below 120kbps to have a good game with Live... Talk with your ISP, they should be able to assist you in troubleshooting your connection issue.
I've played around with the upstream speed, and found that anything really below 100kbps for Live gives you constant "lag" and interupts your game quite often.