What are the dimensions of your room? If you are, for example, in a 10'x10'x10' cube then instead of spending any money on gear, put the money towards rent and move to a place where you have a better room.
Is your room set up properly? Are your monitors placed in some semblance of an equilateral triangle, with the tweeters at ear level? Are they equally distant from the walls, or are they tucked in a corner? Is your desk a reflection point, causing comb filtering? Is your computer relatively quiet, through placement, baffling, quiet fans, or better yet moving it out of the room completely?
Are the first reflection points treated with broadband absorbers? If not,
make some. Then make some bass traps, especially if your room is small.
Once all that is done, consider upgrading your monitors. Until then, they'll do. It's kind of like videography: a mid-range camera with a properly lit subject will look better than an awesome camera that is unlit. Find some marketed as reference near-fields that don't hurt your ears when you play some commercial music you like, and then learn them. Monitors that you
know (in a good room) are more important than monitors that are "good" or "expensive". You may find the Rolands are fine.
Only after all that is done would I even consider upgrading from the MBox 2 Pro for D/A reasons (as opposed to reasons like number of inputs or preamps). As a newbie, you can't even begin to evaluate preamps and conversion (or subtle differences between microphones) until you are in a good monitoring environment. If you keep the A2D you'll have A/D covered and have 2 wicked preamps (I've got an A2D and I love it). And I'll wager that if you are doing a lot of overdub tracking your next target will be a small decent mic collection.