it'll be hugely noticeable. SD is up to 720i,which means 720 interlaced lines (every second line is black on every frame, but because they alternate which is black and which is actual picture they call it 720), or 520p (I think) so 520 lines of actual picture every frame. You have a display that's capable of displaying 1200 lines, so 520's gonna look like crap.
HD, on the other hand, comes in 720p, 1080i and 1080p flavours. These put a lot of strain on your cpu to decode but hell, don't they look purty. Nobody broadcasts in 1080p yet (to my knowledge), but you can download movie trailers in that format to see what it looks like (and it looks like awesome). Then compare it to a dvd (which is 576p, or thereabouts) and see if you can tell the difference.
But yeah, if you're gonna buy a tv tuner for a WUXGA screen, definitely get a HD. Screen size won't really matter, I don't think, it's a matter of picture clarity.