I have a lowly 8500 cheapo card with 1GB, and I don't suffer from significant stutter. Occasionally Vista wants to install a 64 bit driver update, and I allow it to do so...
I have a video with 54 stills (various sizes) moving (in 3 axises)around in the background, with various states of alpha (dynamically changing as the images move) and text sliding out from under the edges of the pics at various moments. Also music playing in sync with the image motions. It is 45 seconds long and is an intro to a video.
I have never seen stutters at all. As a matter of fact I was impressed with how smooth the editing process went with it.
XP has a different driver model for video cards so that could be a factor. Or it could be a bad driver for your card, or a driver conflict (ASIO vs GFX card, for example? Audio drivers are sometimes a culprit and are easily overlooked)
Or it could have been a misconfiguration. One problem people have, I think, is that they're very consistent about how they set up their systems and sometimes they consistently mess things up.
Although probably everyone sees an occasional stutter or stumble, it seems like a lot of people don't have as big a problem as you're having. So either your system is especially troubled or you're especially sensitive to it.
Oh, and on the AJA/BMD front, this is the professional way to go. Most users are happy to skip the professional monitoring but it can become a requirement when you move up the scale a bit.