When you talk about a ceiling, there's a lot of factors to consider.
First, what's the bandwidth of all the components in your box?
Two, what's the load put on your system by the games you're running? (If you play Half-Life, the load will be less than if you're playing Doom3 or UT2k4).
Three, how much RAM can your mobo handle.
Four, how much vRAM does your vid card have, and how well does it run?
And five, how much power do you have? (PS wattage.)
All that will effect your "ceilings."