Should I go with the HD 4870

The card is, I wouldn't bother changing the mobo as it will cost unnecessary money unless you're planning to further change you system (crossfire, overclocking your cpu).
Your current mobo will run the 4870 just fine, make sure you get a good price for your old GTS =-)

It doesn't matter in this case. An aging g80 8800gts has trouble keeping up with the newest games even at a moderate 1280x1024 resolution, which basically all monitors can handle. It's time for an upgrade.

It's pretty far behind. g80 8800gts perform about the same as 9600gt.
http://en.expreview.com/2008/04/02 [...] p/?page=10
It's a g80 card, not g92. They're actually a generation apart, despite the naming scheme. The gap is significant.
http://en.expreview.com/2008/04/02 [...] p/?page=13

If you're going to keep 680i, consider either dual sli two g92 8800gts, now costing as little as $120 ar, and will outperform a single gtx280, far outperform 4870. Or tri-sli 3 9800gtx. It doesn't scale as well, but still deliver better performance, and at as little as $150 each, is more cost effective than using the latest generation cards.

g92 8800gts and 9800gtx run on the same g92 gpu with 128 stream processors. 9800gtx support tri-sli, 8800gts does not.
http://en.expreview.com/2008/04/03 [...] review/12/

Also, the 9800gx2 dropped to $280, outperforms 280gtx, far outperforms 4870, and is now a good choice. It's slower than g92 8800gts sli due to slower core clock, but gives you the choice of quad sli at reasonable price/performance ratio.

Uhm no it doesnt have trouble...my G80 gts 640 is still kicking butt and playing them just fine at 1920 x 1200 all high settings...cod4, frontlines and even runs crysis on high avg 24fps. I am moving to a gtx280 since the price drop now.

Thanks for the feedback, I'm planning on buying a 24'', will that change anything? So I have 3 different oppinions, buy another gts 640Mb, buy the 4870 or 2x260gtx (im thinking this could come out quite expensive) and the final thing on the motherboard is that I shouldnt change it? I was also considering changing Q6600 for Q9550, that's why I took into consideration the motherboard change !

If value matters, you should look beyond just the latest generation cards like 4870, gtx260 and gtx280. 9800gx2 cost $280, far less than gtx280, same as 4870, but outperforms gtx280, and far outperforms 4870.
http://techreport.com/articles.x/15293/8
http://techreport.com/articles.x/15293/5
http://techreport.com/articles.x/15293/6

Quad sli does not scale well. The 3rd and 4th gpu only add average 30% extra performance. For single card, it's the best value though.

I game at 1920 x 1200, and went from a 8800 GTS 640 to a 4870.

In games where it works, it absolutely flies - fantastic performance. However there are all too many games where it doesn't work properly - the driver support is poor - plus the card is noisier than my old GTS and also runs hot. You can cool it down, but then it just gets louder.

My system is designed to run quiet, so it is more noticeable than normal.

Think very carefully before pulling the trigger on a 4870. At the end of the day, they're cheap for a reason.

What to do when you want a multi-gpu setup and you have an Intel chipset mobo? Can you go SLI on an Intel mobo or is there not other way than choosing for crossfire, because Inetl supports it?