Look in this thread for some good info on slot coolers. I point to
another thread here about one person's great idea with a slot fan.
Some slot coolers are decent (ie "alright but not great") for cooling cards. The main worry about them though is if they exhaust hot air out or take cool air in. Video card fans take IN air, so you the slot fan should be blowing air AT the video card.
This would be a good fan to get
I don't blame you for being cautious about warrenties. For a great majority of people, $200 or $300 is no small piece of change. That being said, overclocking isn't the best option for everyone. For those who don't know a lot about it and don't want to take a risk, its better to not even bother with it. Stock parts work fine.
If you're going to stay with stock parts you probably won't even need a slot fan. If your case has poor circulation or your computer is in a hot room, a slot fan wouldn't hurt. I recommend whats mentioned in the threads I linked to. Buy an el-cheapo slot fan and rip out the fan itself. Carve out the fan opening to match with the video card fan. This will allow it to draw in cool air straight from outside the case. You'll have a much cooler computer for only $10 and you'll still have your warrenty
