Some things to consider.
A few key geometry points on a frame / bike are the chain stays, top tube and head angle. The shorter the chain stays, the easier to pop into and out of manuels - benefits street riding/flatland, depending of preference. The longer the chain stays, the more stable the bike - benefits ramp riding / dirt jumping. Top tube length determines overall stability as well, along with just how comfy you feel while riding. Flatland bikes have VERY short top tubes - I'm 5'7 and run a 19" tt. Most street / dirt frames will run between 20.5 and 22.5, depending on your height and preference. Head angle also affects stability. A steeper head angle - like a 70.5 degree or something - is more twitchy, but reactive and spins better, also good for tricks when the bars are backwards. A more lax head angle makes the bike more stable, less twitchy.
For components, you want to make sure you get double wall rims. also, i'd run a cassette, not a freewheel. It's just more new school and you'll have more options in the long run. Make sure you get a full chromo frame, too. Some frames have a chromo top and bottom tube, while the rear triangle is hi-ten steel. It's not bad if used just in the rear, but it's heavy as hell and you're best to just go full chromo. Stay away from aluminum for anything but chain wheels, and I'd still stick with 7000 series even for that.
Some "better" brands are Sunday, Odyssey, we the people, s&m. Mid tier would be like stolen, SOME eastern stuff, Verde, cult. I stay away from fit, haro, mirraco and SOME hoffman stuff - though, Hoffman makes the strowler frame, which is one of the best straight up, all purpose freestyle frame on the market for the money.
Check out albes, bmx guru and danscomp for online shit if you don't want to shop local. Pink bike, vital bmx, bmx museum are good resources for info / talk, too.
