Actually, most GF4s do not have any heatsinks covering their ram. The only two I know of off hand are the Gainward and Asus models...possibly the Creative model as well(can't remember for sure).
Even then, the ram heatsinks are really overkill unless you plan on doing some rediculous levels of OC on your GF 4. The reason behind this is the design and manufacturing process behind the RAM on the GF4 ti boards. The ram, made by samsung I believe, according to the tech specs allows the ram to reach levels of 750mhz before requiring a heatsink. As it stands I don't think anyone has been able to push their ram past the 735-740 mark anyway...at least not yet.
But yeah, heatsinks are going to be on at least a couple of the models coming out soon by other manufacturers. I again can't rememeber whether it was Asus or Gainward but one of those two actually has this pretty crazy heatsink that actually wraps around almost the entire card and features two fans..one on the ram area and one on the core area. So yeah they are coming but on the cards out now, they are not needed cause you can't hit the levels where you'd need it yet.