How would it look if I paint the top of the fireplace

It would look fabulous! Maybe paint it black! Just clean the surface, lightly sand, clean again, and paint! IF it is oil paint on the surface, then use a primer first, or just paint it with an oil paint. IF however, you want to switch to an acrylic paint, (if you can) buy from Porter Paints (PPG), buy Advantage 900 in a semi gloss. That will work over oil and has superior adhesion. Make sure you let it dry out several days before setting things on it.

That is very personal,however it would give it some definition and character

That'd need to be your call I recon since you will be the person living with it. Maybe strip it to its natural color or an accompanying color of the fireplace.

Just great! It's only paint and if you don't like it you can paint over it. Live a little and have fun!

You can even apply gel stain over the paint if you would like more of a wood look. I've done it. Do a little at a time until you get the hang of it.

Just clean with steel wool, wipe dry, and apply stain with a sock, or rag, or brush on in one direction (the length of the mantle).

http://www.lucydesignsonline.com/2011/05/ive-been-to-gel-stain-hell-and-back.html

I think painting it would be a good idea. I don't think I would do black because that matches the tv. I think a colonial blue would look nice, but I can't tell what colors are in your room. Either match something or do a contrasting color.