A ? for Mapmakers w/ Tiles

I've read that it's better to use one single image than multiple tiles, layered or next to one another.   For those of you who have experienced lag, or memory issues, about how many tiles does it take before problems begin to start?

There's actually a change on the dev server (dunno if it's live or not yet) that makes tiled maps viable now. I have a map I've been tinkering around with that has at least 300 tiles and objects and appears to render very quickly. I'm still adding to it and I also use dynamic lighting on it. I have yet to run a game with anyone else on the map yet though... I may try to run a pick up game here soon just to see what the performance is like for other people.

To my understanding it's (the update which changes how the server handels objects) not live yet.