I wouldn't change the fan on your videocard. I've never did it before so I can't give ya any advice. There are fans that are designed for NVIDIA and ATI cards but again, I have no advice to give. The default fan should suffice!
The GeForce 6600 GT has GDDR3 RAM on it. This makes up for the lack of a 256-bit memory interface. The 6600 GT is faster and more powerful than the 9800 Pro but the 9800 Pro is an INSANE deal.
The games you listed are compromised mainly of EA's Battlefield series (which I enjoy to an astronomical degree
). I hate to say it, but ATI's cards simply slaughter NVIDIA's in the BF series. Faster framerates, better image quality, they simply run the BF games better in every aspect. If you're gonna be playing the BF games primarily, I'd suggest a Radeon 9800 Pro. Make sure to get the 256-bit version!
Oh, and there aren't any brands to watch out for. Powercolor used to have a bad reputation but they redeemed themselves with their excellent 9800 Pro card.
-Lasereth