I generally suggest 5000 big blinds for the average player to withstand any major downswings of bad luck. With only $500 you'd have to play $0.05/$0.10 if you wanted to stay conservative. But if you're willing to gamble a little with the $500 and say a little prayer to the god's for some rungood, then you could take your chances at $.25/.50, but realize that is only 1000 big blind (50 short stacks), and can all be lost in one major downswing. However, to keep things in perspective, 5000bb is generally the recommendation I have for mid/high stakes NL. At small stakes NL you can probably get by with less since the games are far softer and variance is generally quite a bit lower in comparison.