Today, theoretically the equipment doesn't mean so much anymore. The quality for a cheap interface compared to the most expensive PT interface isn't that much. Or at least to normal people ears. What it depends on is that you can do the art of mixing/mastering. Nice tricks is to have equipment above a certain standard in resolution and really good preamps. Other then that it might depend on plug-ins/processing units and etc. But the computer and digital plug-ins have a lot of nice features. If you take a really good mastered album and play where you're going to mix, on the same monitors and computer, etc. Maybe the good mastered album needs more bass or having too much middle, then you compensate to it and compare your master with the good mastered along the process. Maybe export a cd or two and lsitening to it on other devices. Then you can compensate the cheap monitors to the (most likely) expensive monitors the great band used mastering their great sounding record. It's all about experience and tips, you have to understand and hear the art of mastering before you can make a great sounding record.
So, a little summary:
- You don't need very expensive gear, though it should be above a standard.
- It isn't the gear that makes the mastering good, it's you!
- Compensate by listening to your favorite mastered and compare it to your master several places and you might get shocked over how few details that can ruin the whole picture.
Hope this helps : )