Make sure your townhome is immaculate before you sell. Definitely take care of any small maintenance issues. Buyers focus on kitchens and bathrooms so if you do any renovating, these rooms deserve special attention, but don't "over-improve." Take a look at other townhomes in your area as they come on the market to see what your neighbors have done with them. This should give you a good idea of how well your townhome will stack up to the competition when the time comes. The "best" unit at the "best" price is the one that sells first, so that's what you should strive for. You should also plan to paint throughout right before you put it on the market. Another nice and relatively inexpensive DIY way to improve your townhome is to customize the closets. The Container Store has a huge sale on Elfa closet organizers every year around the beginning of the year, when they discount the entire line about 40%. Every year for several years I picked one or 2 closets to work on and have since customized every closet in my home. It's a nice amenity -- especially if you don't have an over-abundance of closet space.
As to using the same agent to sell and buy. There's no major downside as long as you can find an agent who has true expertise in all these separate areas. If your current townhome is in 30341, then you're really not very close to either Underwood Hills or Oakhurst. The vast majority of agents in Atlanta are specialists, rather than generalists. As you can imagine, Atlanta is an enormous metropolitan area and it is simply not possible to have in-depth expertise about every neighborhood. The best agent to sell your home in the Chamblee area may not really have any depth of knowledge about either Oakhurst or Underwood Hills. They will certainly be glad to show you homes in any area and will probably do an adequate job, but neighborhood specialists generally have their "ear to the ground" about properties in their area and will often know about properties that might be coming up, know about special circumstances that might give you a better deal on a house, and know about upcoming changes to the neighborhood that could affect property values in the future.
I know several agents who are genius with westside properties and many agents who are genius with eastside properties, but I know none who are truly genius with both. I think you would be best served if you narrow your search to either eastside or westside. Spend enough time in Oakhurst and Underwood Hills (and the surrounding neighborhoods) to make your decision. Then find an agent who can help you find your new home.
For your listing agent, I would encourage you to look around during the period before you sell your property and see who are the most productive agents in your area. Take down contact information from signs and see if you can find a pattern of an agent or agents who get a lot of listings. Then figure out whether or not that agent is successful in getting those listings sold in a timely manner (number of listings doesn't necessarily guarantee quality in an agent). Include that agent (or those agents if you find several) in your interview process when you go to sell your townhome. The most important thing to discuss at that interview is the marketing plan your agent will put in place for your property and the timeline by which the marketing plan will be implemented. Listings get the most "buzz" in their first week or two on the market so it's crucial to get marketing in place up front (preferably before the listing goes on the market). Also at the interview, question the communication methods and philosophy of each agent. How do they communicate with their clients and how often? Lack of communication is one of the most common complaints that sellers have about their agents. If this is important to you (and it should be) find out what you can expect from each agent. Finally, you need to feel comfortable with your agent. If you don't think your personalities match well, then choose someone different. Selling property is stressful. There's no need to add to it with personality conflicts.
You should include in your interview the agent or agents you're considering as your buyer's agent in the neighborhoods where you're looking. Agents will frequently offer some kind of discount on commission if you let them both list your current property and sell you your next one. The commission break with typically come on the second of the 2 transactions. If one agent is suitable for both transactions you might save yourself a little money by going that route. However, the commission discount is not a good enough reason to choose one agent if he or she cannot adequately assist you in both transactions.
Best of luck!