There seem to be a few variations on the card, but the most common (and the one that I can see as a likely one for Medion to fit as standard) is 256MB. If you have the model of your computer, you could try Googling it to see what is fitted.
As for being any good, a hardcore gamer/video editing person would laugh at it but for most normal use I would say it's fine. I'm not heavily into games, but the 2 newest games I have - Bioshock and Assassin's Creed - ran fairly well on my machine. I think I had to turn down a couple of the settings on "Creed", but it still looked pretty good to me. I do a bit more than what I assume to be the average amount of video editing and haven't had any problems there either. Photo-wise, I also do a fair bit (but not to professional "Photoshop" standard) and haven't encountered problems. (I too have a Medion machine).
Unless you're having difficulties with what you have, I'd stick with it. Graphics cards are easy enough to replace if you do decide you need to upgrade at a later date.