Covering a DIMM slot isn't an issue, the height of the RAM fins is what you need to worry about. Pretty much every decent cooler now will hang over the memory area so you can't get RAM with tall spreaders.
The Ripjaws series don't have fins, only a little bit that sticks up, so you'd be fine with pretty much every cooler out there - it's not tall enough to get in the way of the cooler or the fans.
So, as asked above, what's your case and how far do you want to push your overclock? These factors will influence your cooler choice, but your RAM isn't going to be a problem.
Kingston HyperX or Corsair Vengeance is a different story altogether (Google some pictures to compare them to the Ripjaws and you'll see what I mean).
Overclocking Sandy Bridge is easy because Intel have nerfed the hell out of it - set CPU multiplier, choose RAM multiplier, maybe fiddle the voltage a little. Done, because you can't really do much else.
For 4GHz I should imagine the Hyper 212+ will be sufficient, anything faster will need something heftier like a Megahalems or those twin-rad monsters like the Noctua D14 or Silver Arrow to be safe.
All will be fine with your Ripjaws RAM as it's not tall enough to interfere.
Yep! Ripjaws and the new X models are 41mm tall.
I would consider coolers that aren't wide for best results with all dimms filled.
Excellent coolers that fit that criteria width wise.
CM 212+ = 80mm
TR Venomous X = 63mm
Prolimatech Armageddon = 50mm
" Megahalems = 74mm