As long as the motherboard has PCI-E (which is does) and you have an adequate PSU for this GPU it should be fine. Judging by the fact you have a 9800 GT which by what I've seen consume a lot of power, the PSU should cope fine with a less power hungry card.
Yes, it should be fine the only worry is if you have room in your case. I don't think this card is overly large so you should be good there as well. You need to remove the old driver before you install the new one. You can do it as clutchc suggest, or boot up in safe mode after you install the new VGA card and then remove it. Reboot and install the new driver.