How well does a 9800 Pro overclock compared to 9800np?

I know you can soft-mod a non pro into a pro.

But shouldn't the pro have more headroom for overclocking? How much of a difference does it make?

From my readings and experience, with a pro you may average 15-20MHz higher on the core and memory, when compared to a np with 3.3ns Samsung memory. However, if the np has infineon memory, the pro will probably beat it by 40Mhz or more. Then there are a few np cards out there with 2.8ns samsung, and they seem to overclock just as well as do the pro cards, since they are using the same memory. Like I said at the beginning, the core overclocks on the pro seem to average only slightly higher than the np.

How can you tell if a card has that memory?

And I couldnt tell if you thought the sam 2.8ns or infineon ram was better.

You can tell by reading the printing on one of the 8 BGA memory chips on the card. The order of memory choices from best to worst is 2.8ns Samsung, 3.3ns Samsung, 3.0ns Infineon.
If you have Samsung memory, look at the last few characters of the second line of text on one of the chips:
xx33=3.3ns
GC2A=2.8ns

Who sells cards with the 2.8n Samsung? Obviously you cant tell with retails since they have the heatsink on them.

I have not seen any retailer guarantee 2.8ns memory. I think it's just luck of the draw with the nonpro.
I also have not seen any 9800np, retail or oem that has heatsinks covering the ram, so if you can see the physical card, you will be able tell what type of memory it uses.

ive heard that all sapphire 9800np's come with 3.3ns samsung memory. that'd be your best bet.