As you can appreciate, this can be a difficult question to answer because it largely depends on what you want to see and how much you want to spend.
There are two schools of thought...(I've exaggerated a bit)
#1 Buy as many different cheap eyepieces as you can until you get a box full and then buy and sell eyepieces as you "grow" in the hobby.
or
#2 Buy few quality eyepieces that you will keep a lifetime.
http://www.televue.com/engine/page.asp?ID=131
I've attached a link to the Televue website and an article about eyepiece selection.
I tend to be of the opinion to buy a few of the best that you can...
But, I'd target 3 eyepieces low power (you have), medium power and high power. Some people would also advocate that a good barlow is important and it can effectively "double" the magnifications that are achievable with your existing eyepieces.
Before you buy anything, I'd suggest that you find someone near you and get together for a viewing night. This is a great way to try different gear in your scope. Depending on your strategy (#1 or #2) you can save yourself a whole heap of "learning" by trying gefore you buy.
Once you know what you want and how much you want to spend...there are many good options available for a wide range of budgets. However, generally speaking, you do get what you pay for...
Good Luck and Clear Skies!