You need about 512-1024mb/s speed for your 3G dataplan to be able to use YouTube without a hitch on your N900. These speeds would be shown by speedtest.net as 0.5Mb/s and 1.0Mb/s respectively.

The reason that the mentioned N97 can stream videos from YouTube a lot better, is that the video you see on that device is of significantly less quality, at least in the way of bitrate and so on. You might not be able to notice the lesser quality because of the smaller size of the screen and/or smaller playsize of the video. The N900 pretty much drags down content from the web "as is", meaning that the videos are of a higher quality and thus requires a faster speed.

I suggest you request an offer for a faster data plan from your operator.

Another thing that affects your 3G speed is the available network for your phone. If you don't see a "3G" or "3.5G" next to the bars that show reception, you're not in an area that supports fast enough use of YouTube without the buffering.

Sorry if you knew a lot of this, hope some of it is useful to you.