The thing you want to keep in mind about your goatee is that the mustache half of it is relatively sparse compared to the beard half, so keep the beard in line with the mustache. I like the idea of keeping it trimmed down.
Also, when you get a haircut, get a style. What you have now is basically "generic haircut A"...if you understand my point. Keep in mind you have a long tapering face, so don't get anything too big/poofy on top.
Here's what I would do. Get your facial hair in order before you get your haircut. Go to a hairstyle place (like Supercuts or something) rather than a barber. Find an older woman stylist who looks like she's been doing this for a while or ask the girl at the counter when you go in who has a lot of experience. Tell that stylist that you want to get/update your style and ask her to recommend a few things. For me, I like to see pictures, but I'm a very visual person.
Don't be afraid to discuss and ask questions.
Don't be afraid to lose the sideburns if they don't work with the new style.
Make sure to keep in mind the daily maintenance that will go along with the new style. I hate daily upkeep (gels, etc), so I keep my stuff short and have even figured out a way to integrate not having to shave every day into my style (but my facial hair doesn't grow terribly fast). I just recently got a Conair Chopper 2-in-1 set and it's the best set I've had in a long time. It does hair and beard with the length attachments for both.
http://www.macys.com/catalog/product/in ... 77-_-MP677
Remember, just the simple fact that you're putting this much thought into your appearance means you're already two steps ahead of most guys.
Also, be sure to post those pics...I'm interested to see the outcome.