There's not really a standard answer for this, as it will depend on the price at which you can acquire varying amounts of different strike calls.
The safest play would be to buy one $5 strike for about $500. If the stock goes to $9, your option would be worth about $400. If the stock goes to $40, your option would be worth about $3500.
For most similarly-priced stocks, the differences in price between a $20 strike and $30 strike are likely to be small in terms of actual dollars. Hence, commission costs will have to be factored in order to determine the optimal use of $500.
You may be able to buy 50 20's for about the same net cost as buying 75 30's, yet you would make more with the 20's if the stock closes at $40 on expiration day ($10,000 vs. $7,500, both approx). Also, the 20's have a better chance of being worth something should the stock only do about half as well as you predicted.
I will occasionally swing for a homerun like this with a small amount of money such as you have indicated, $500. Very rarely has it worked out (though not never!)
FWIW, I'd probably buy something like a $12 strike 2 months out, and be watching for an opportunity to sell a current month contract OTM - perhaps a $13-$15 strike as time was winding down. Of course much would depend on the particular situation.
If the guy recommending buying $40's is still around, I'll sell him all he wants! Most likely for only $0.10!
AM.
