Well, if you take the digital out from your MBox2 to the digital in on the 001, you have enough pre's do do a drum mic setup, with 2 overheads and one out front. With that said, you'd need two of the same mic to run as overheads, so I'd start with another SDC, whichever of the 991 or SM57 that you like better. I don't know if you can get the 991 as a single, it might only come in a pack with the 990, but since that pack is about the same price as a new SM57, the choice is yours.
So, $100 to either a 57 or a 990/991 pack. You can now record drums if you want.
A nicer preamp would also definitely be the thing to do. If you could scrounge up $100 more (we have $400 left after mic purchase), you could get the Presonus Eureka new ($500). Right now, they're also throwing in the A/D card free if you purchase by Dec 31. With the card, you can use the (probably) better conversion on the Eureka and go right into the digital in on either of your interfaces. Another option is the RNP, also about $500 new. Either would be less used, obviously, so you can also go that route. If you do get the Eureka used, you won't be able to get the D/A card, though and that normally runs about $200.
All in all, your price range for mic pre's is a little tricky. Anything much over $250 but under $1,000 usually isn't worth the investment (there are a few notable exceptions), so you may find your options limited here. That's not to say that there aren't good pieces of gear out there. The RNP and Eureka generally do well in this range. There are other units out there, too. Dig through these forums and see what others reccomend.
If you could afford it though, the above is what I'd shoot for, based on your simple overdub needs and occasional desire to record drums (recap: either another SM57 or 990/991 mic pack, and then a carefully-selected preamp.)