Which is faster?

1 1024x1024 texture or
4 512x512 textures?

I know surface count *should* be the deciding factor here, but when I merged (baked) all my textures into one big one, there *seemed* to be a slowdown :/

Could just be my imagination though - I haven't done any conclusive FPS tests - so I thought I'd ask before wasting my time ;)

What was the graphics card used?

Smaller textures are probably the fastest, even if you get more surfaces. I had a situation once, where it was faster to use 16 256x256 textures than a single 1024x1024. That was faster on both a GeForce2-GTS and GeForce4-Ti4200.

I thought it might be, but I've been brainwashed into believing surface count is everything ;)

I'll have to go back and redo it all now :/

Well, before acting on "seemed" and gut feelings, do some measurements.

I second (or third?) that. Often on older hardware, no even that old, big textures are a slowndown. And since if you wanna make something that will run on a fair number of computers, i shy away from using textures that size.

It's bandwidth and card memory. Please don't go over 512x512 for mainstream.