you're not lacking in the pre department so I'm not even going to get into preamps.
the mics could be better, but should do.
these are the things I would worry about:
1. Tuning the drums well and using new heads. check out the drum tuning bible here:
http://home.earthlink.net/~prof.sound/
2. The drummer needs to be able to play consistently. Don't take this to mean that dynamics are the devil or that you need to record to a metronome. I just mean the drummer should play like he means it. Speeding up and slowing down is ok, but it needs to sound intentional. Playing louder or softer should sound intentional and fit the song also. For a good recorded sound the drummer should be playing in his comfort zone. Meaning that if they can't consistently play those single foot doubles or triplets they just learned and want to put into every song, they should either dumb it down, or go home and practice until they can. You'll never be able to fix these sorts of issues in the mix without resorting to replacement, beat detective, and all other sorts of trickery.
3. The room would be next: Check out ethan winers site about acoustics here:
http://www.ethanwiner.com/acoustics.html
Make sure you really understand those first 3. They are the most important part of getting good drum sounds.
These next things will have an effect on the sounds you get, but not nearly as much as those first 3 steps.
Your mic placement is probably the next most important.
How you treat the drums effect wise (eq, compression, reverb, etc.) will have a pretty big effect on your sounds also. There is a ton of info on this and many other bbs's about mixing drums. All you need is to use the search function.
Converters-- this would personally be the last thing I worried about, but eventually you will probably feel they are the weakest link and will need upgraded.
Eventually you may want to invest in some good eq, compressor, and reverb plugins, or hardware. For the moment though, there is no reason you shouldn't be able to get good sounds with the stock plugins that came with your daw program.
A thing that lots of people forget about, or don't know about is that every plugin you put on your tracks causes a delay on that track. Many DAW's don't compensate for that delay, so if you're mixing drums the track will play back slightly later than you intend. It can cause some phase issue's, especially if you're using an aux track for parallel compression.
You can figure out how much each track is delayed and physically nudge it back in the session so that it plays back where it originally was supposed to play.
Before anyone tries to do the following things, the most accurate way to do it is to switch from minutes/seconds, or bars/beats, or however else you may be seeing your time in your session to samples.
The easiest way I know to check how much delay there is on a track is to assign the track to a bus and record that bus to a new track then figure out how much time has elapsed between the beginning of each track.
The bus will add a certain amount of delay also and you don't want to overcompensate by dragging your drums too far back, so you should figure out how much delay the bus is causing by bussing a track without effects on it and recording it. Then check how much time has elapsed between the beginning of the first track and the bussed track. Subtract this time from the delay on the track that has effects and that is how far you need to nudge the original track to get it to play where it was originally intended.
