Dirt riding can only take you so far. Its not bad experience, but keep in mind that there are skills that mostly apply to dirt (like scooching your ass all the way up to the handle-bars and sticking that leg out to turn like a mofo), and likewise skills that mostly apply to street (countersteering, the do's and dont's of trail-braking)

Drftr doesn't sound like he knows much. With riders of equal skill, a GS500 would completely school that wide-as-hell, pig-of-a-bike-98 GSXR-600 on a tight track.

If you are looking to spend $5k then do this:

•Take half of it and put in the bank for your SECOND bike. Spend ~$2k on a starter bike and $500 on gear (helmet, gloves, jackets.... pants and boots if you feel like it).

•Learn to ride your bike, and get as many test rides on as many different bikes as possible.

By time you are ready to upgrade (usually 4-6 months, depending on how much riding you do) then you'll have:
1. A better idea of what you want to get (after having ridden different bikes)
2. More money to play with

You might even decide to keep your learner bike, get some leathers and turn it into a track-bike.