If you play a smart loose-aggressive style, multi-tabling is going to be difficult, because you'll be involved in so many pots at one time. But for tight-aggressives, it's pretty easy to play 4-6 tables at once with a little practice. Even if your winrate goes down, the added hands should more than make up for it.
For example, you're winning 5.7BB/100 playing one table. If you gradually work your way up to 4 tables, even if your winrate drops in half, you're still making twice as much $ per hour as you were. And, if you're playing too many hands, then multi-tabling might actually increase your winrate, as you won't have as many boring lulls that can convince you to get involved with marginal holdings.
So, I'd add one or two extra tables, and keep grinding at 10NL. Then, when you're solidly bankrolled for the next level, you can drop back to one table and move up.