If you want to SLI 2x 9800GTX's, you're better off doing so on a 22" and higher.
Otherwise, it would be over kill on a 19". The max most 19's support is 1280x1024. However, I've seen one 19" on newegg that supported higher then that.
Like what was said before, you could go 2x 9600GT's. That'd be a great combo, it's what I'm running right now, couldn't be happier with it.
SLI'ing high end cards is a compleltey different story from SLI'ing mid-range cards, like the 9600's.