Okay, I've googled this to no avail, but I figured SOMEBODY would know it

I have a PS3 hooked up to my PC monitor via a VGA box and want to add a capture card to it for Youtube. Is that possible (and if so how?)

Not exactly sure whats the question. To know if capture card will work? Yes it will, but consider that - it will eat almost as much power resources as fraps does, so think again. Maybe its easier to use fraps instead.

Basically

Is it possible to hook a PS3/X360/etc up to a capture card, then up to a PC monitor?

Yeah, thats what capture cards are for, right?

But u wont be able to play realtime trough monitor. Well, u can but u will have about ~1sec. lag in this case - depending on the card and connection.

Edit: U will be able to see the game trough monitor with a PCI card. But with an external capture card (example: Hauppage HD PVR) u must make sure it has enough outputs. This particular Hauppage has inputs, and multiple outputs. If the capture card doesnt have enough outputs, u can just split the signal but in this case u need an amplifier. For example, u will split a HDMI signal - u need to amplifer, else way the strength of the signal is 50%/50% and the picture loses quality and will be very dark.

Yup, I agree, I've tried this a long time ago with a PSX. I connected it through a capture card and it was unplayable. The lag between when I pressed a button and when something happened on the screen was immense, like 1 or 2 seconds. So it's not possible. Unless you're able to record a replay on your system, then connect it to the capture card, and play the replay through the capture card, then it would work.