Quite good for using just Anvil for all these! Also, I take it you're using the MSGS default synth, and maybe no accessories included.
I would also suggest replacing the instruments with VSTis, or contacting those of us with extra resources (including myself) to remix them with better instruments to appease the "MIDI music is cheesy" population.
The other MIDI enthusiasts/masters should be coming around soon.
(Note: I compose all my songs with Anvil Studio. It's a free midi composer, a lot easier to use than, say, FL studio.
It's a good tweaking program but for more serious business I'd gradually transfer elsewhere. I started with Anvil as well, but as my workload and requirements went up I've since moved to Logic Studio and Sonar X1 Essential for my main stuff.
When I'm finished, I use a midi to mp3 converter which can very easily be found on Google to up the quality a bit.)
Oh the irony. MP3 is a very lossy format and not good for mixes.
If I were you, I'd rip the MIDI tracks to 192kHz 32-bit WAV files for use as a conversion base to MP3, OGG, M4A, FLAC, etc. Despite the massive file size, creating a lossless file mix is more than worth it and allows for better audio precision and accuracy in playback. Plus it prevents the terrible lossy to lossy-lossless format issues.
Of course, for distribution lossy formats are okay, but even in those cases try to maximize their quality (no 128kbps rips, I DO notice and I'm no audiophile).