"for a price of $5" if that means "i'm using a limit order" and you
set $5 as your limit you won't pay a penny over $5 for any shares.
You also might not get the order filled (if the stock jumps up before
you execute to 5.01+) you won't purchase until an asking price drops
to $5.  If the lowest asking price is $4.98 when you set a limit at
$5, you'll get 4.98 (which is better for you).  If you mean "what
happens when the stock is at $5 and I place a market buy order?" --
thats different.  That will fill all 500 shares at whatever the
current asking prices are.  For stocks changing value fast -- this
means the price could jump.

If you check all-or-none then your order will only fill if you can get
all 500 shares at the same price and in the same transaction (by your
broker).  I usually don't check it, and use limits, which means on
larger orders (like 1000 shares or so) i'll see 3 or 4 sets of shares
in my account afterwards.  150, 100, 200, 550 say -- and they will all
have the limit price or lower.  all or none increases the chances that
your order will not fill.  But only if the share volume is large (500
isn't really large -- neither is 1000 really...).  Even if it takes 3
or 4 transactions on the part of your broker -- you'll pay only 1
commission for it.