Wavey lines aren't caused by overheating, and video cards are the most resistant to heat, it is normal for most to be operating at around 80 degrees when gaming or doing graphicaly intensive stuff.
If the card has a passive heatsink (no fan, just a hunk of metal) make sure there is airflow over it, if it has a fan, do as bomberboysk said and download EVGA precision and increase the fan speed. If that doesn't make it as cool as you would like it to be you can always buy an after market heatsink, you can get some cheap ones that are pretty damn good, arctic cooling accelero for example, it is massive so it won't fit in anything smaller than a mid size case, but it is very cheap compared to some of the others, and keeps the card cool, and is passive so silent and because of the size you don't NEED dedicateed airflow because the airflow in the rest of the case, even if it is small, happens over a huge area so acts almost like a dedicated airflow on a smaller passive heatsink.
The wavey lines will be caused by one of two things:
1. your video card is on the way out, but as it is 9800GT i am guessing it is fairly new and as it is xfx, that shouldn't be the case
2. Your PSU isn't good enough for the card. If the PSU is the one that came with your case or one that cost you $30 then chances are even if it says it is a 750W PSU, it isn't, it will have low amps on the 12V rails and therefore not be able to power your rig properly, so you aren't getting enough power when you need it
. That one blew quite spectacularly, flash and bang and insides of the unit messed up