Their is alot of variables in that,such as what type of 9800gt it is and the specs of the 17" monitor, but for the monitor being $100 you can get a 9800gt on newegg for $70 after MIR.But other 9800gt's go for around $90-110.Depending on what type of 9800gt it is i would say it seems to be a fair trade.
The HansG monitor is a cheapy anyways, a 9800 is still a powerfull card. Although a bit out of date, it can still play many games at a great resolution with high fps. I would do it! Except I don't need to, woot!
I'd stay away, not that it's not a good deal. You just have no idea what has happened to that card (cooling issues anyone?), you might only have that card for a week before it dies (since it's a little older). The same thing can be said about the monitor, but at least you own that so you have an idea of how much life you'll get out of it. That's the honest opinion.