D-SLR on a budget

Agree with all the advice above. $500-600 is very limiting for a D-SLR. You likely will need to buy a used camera.

Here is a decent starter kit:
http://www.bhphotovideo.com/c/product/800693372-USE/Nikon_25420_D40_SLR_Digital_Camera.html
http://www.bhphotovideo.com/c/product/486717-GREY/Nikon_2166_AF_S_DX_VR_Zoom_Nikkor.html

If you want something more powerful:
http://www.adorama.com/US%20%20%20%20383142.html
http://www.adorama.com/US%20%20%20%20382491.html

New equipment will cost a little more. This Pentax Kx kit is a little over $700:
http://www.bhphotovideo.com/c/product/651673-REG/Pentax_15801_K_x_Digital_SLR_with.html

You still need to save some money for a bounce flash, tripod, filters, etc.

If you are likely to upgrade to higher pro-sport level performance equipment or full frame cameras, then Canon and Nikon are probably better choice. You can find accessories more easily and more readily available for these two manufacturers than others (eg. tethered shooting, etc).

Pentax generally has solid quality and very reasonable price.

Good luck.