Get 512K. That way your downloads will be faster, so will your web surfing. Altogether a much better experience than 256K.
The max content you'll get will be 4GB anyway. Beyond that, you may have to pay extra per MB or your speed may be reduced to minimum if your ISP has a fair usage policy applicable on your current plan.
Remember, the 4GB counts towards not only the actual downloads you make (music, video, pics, apps) but also every webpage you surf.
Normally, every ISP would have a website that'll let you monitor how much you've downloaded till date. You can also use apps like Netmonitor.