I have a 9800 gt on the moment and a 17'' monitor. Is it worth going for a upgrade or maybe sli. Or is the 9800 gt fine for now. I am quite a gamer and I want to play games at high,not very much worried by ultra or very and that stuff.
Do you have to upgrade? No. Is it fun to upgrade? YES.
I would really point you in the direction of a new monitor for your next upgrade. If you are "quite a gamer" then it is quite nice to stare at a nice big monitor for hours on end. You can snatch up a 22" pretty cheap these days. However, at a higher res your card will start to struggle a little more. You mentioned SLI, if our board supports SLI (2) 9800GT's run pretty darn well in pairs. You can snatch one up cheap on ebay.
There is no real need for the second 9800gt with your 17" monitor but you would be ready for a monitor upgrade for sure.
Visually I got more of an upgrade when I changed from 17" to a 22" at 1680x1050 than any GPU upgrade I have done in the past.
With the resolution on your 17" I can't see any reason you "NEED" to upgrade at all.
I have to agree with Jay2Tall though. A monitor upgrade (and then a GPU upgrade) will get you good performance, and a much better picture quality.
If you buy anything new today though, don't do 1680x1050. Go for an actual 1080P (1920x1080) monitor. The 1680x1050 resolution is quickly fading out as 1080P is becoming much more the norm/standard with HD content becoming 'more' mainstream these days.
You could get a GTX 260, or ATI 5770 to perform fairly decent @ 1680x1050 or 1920x1080 with most current titles.
Time to move off that monitor. I had a 17" monitor around the start of this year. Gamed at 1280x1024, no problems. Around Mayish I moved to a 20" LCD at 1680x1050. I wouldn't say its been awesome but its been nice. Perhaps the best improvement that I've noticed is the less heat coming from my monitor/station. That and watching TV/Movies is a lot nicer in widescreen. Unlike my dad I'm a huge fan of WS. If you are at 1280x1024 or smaller, you don't need another 9800GT. Heck even my 8800GS handles TF2 maxed out (2x AA though) at 1680x1050. And your card is better then mine.
Your GPU is more than powerful enough for your 17" display.
Spend you GPU budget on a new 22"+ display, it is a much better upgrade.
Even at 1650x1080, your current GPU is quite decent, especially as you do not care about running everything at their top settings.
Beyond this, a better understanding of your other components would be helpful.
2x 9800GT should give you similar performance to a 4890 or GTX 275.
It will defiantly have no issues at 1680x1050 and will make a darn good showing at 1920x1200.
If you do not mind playing with slightly lowered settings in some newer games, I would defiantly recommend a large 24" display.
Also, I would recommend upgrading to 4Gb+ of RAM.
2Gb is not enough with today's games and software.