Yes you will see a big difference between the different RAM speeds,
usually the core speed doesn't make that much of a difference as long as not like 100 Mhz in either direction...
But the RAM is the difference between 6000 3dmarks, and 8000+ 3dmarks.
I would suggest you go with an oem version.
Because they are about $100-$150 cheaper than retail, and overclock better than the LE
and they can be overclocked fairly easy with Powerstrip.
I am running my OEM at 282/282 Core/Clock
that's a 32 Mhz or 13% overclock
I have 4 Ns Hynix Chips on my card, but if you can find an OEM with the 3.6 Ns chips, they overclock better
without the voltage mod than the 4 Ns do without the Vmod.
(with the Vmod, its much closer)
But, on the flip side, the warranty for the OEM cards is usually though the seller of the card,
and it's like 30 days,
as well as mine did not come with the DVI-VGA connector, which I really wanted
Just my $0.02