So what card did you end up getting? The 5670?
^ That's not a bad explanation of RAM. 4gb is a good amount usually. I just upgraded to 8gb, but it's pretty rarely that it's gone over 4gb (I've used up to around 5.5gb once lol).
Generally speaking, if you got the 5670, these ATI cards are pretty decent overclockers. You can probably push another 10 fps out of your card. There's 2 considerations tho, the first is temps and the second is voltage. Firstly, raising the clocks can increase the temps, and you don't want to push the temps too high. Secondly, the maximum clock will be limited by voltage (if not by temp) but not every brand of card will have voltage control. Plus, more voltage means more heat.
If you want to OC your card, I recommend not using the ATI Overdrive utility withing CCC. It's not terrible or anything, but it's not that good.
You should consider getting MSI Afterburner. It can control gpu and memory clocks, if the card supports voltage control it will allow that, and it allows you to create custom fan profiles so that the fan will ramp up in speed with the temps that way you can have a super quiet idle but when gaming still keep the card cool enough. Lastly, the program comes with pretty good monitoring of all things GPU - FPS, temps, fan speed, clocks, voltage. Lots of stuff! It's pretty easy to set up, if it's something that interests you, you can PM me or quote me here so I'll see it.