Ram ns is access time in nanoseconds, (a nanosecond = a billionth of a second), so the lower the number the faster the Ram.
5ns is faster than 5.5ns.
You should get a better overclock with 5ns ram.
The Pro label for GF2 cards was originally supposed to have 5ns or higher, (according to the nVidia reference specs). leadtek and Evga confused the issue by making cards called Pro's that had ram lower than the reference. Just search in the video cards foum for "Leadtek Pro" to see some of the topics.
I'd recommend If you can find the extra cash get a Geforce3 Ti200 as it will last you longer, be faster and most overclock to original GF3 clock speeds and some get near or above Ti500 default speeds.
No problem, don't forget come back if you have any trouble with installing / setting up the card.
With 5ns ram you should get at least 430 mhz on the ram and the core at least 230. Get it set up and runnig right at default first, then just go up with the memory slowly until you notice artifacts in games or in locks, set it down to the best speed below this. Once you found the highest ram speed, go you with the core a little at a time until again it locks or artifacts. again set it down a bit and you've performance tuned it best you can........ enjoy