I'll give you a big tip-you can't put a 1200MHz on your motherboard. There. If it's an 800E not an 800EB you might be able to overclock it to 1066MHz by raising the bus speed to 133MHz. Then again maybe not. A cheap way to go is to trade your processor for a 700, then overclock that to 933, this one is fairly easy and should be free. If you don't overclock, get the 1000EB.
The GeForce 3 will give you a lot more features but not much more performance in most games. A few of the newer ones, such as Max Payne, will take advatage of those features.
Looking at your current specs, a faster processor will give you the greatest speed increase for games, but I wouldn't even bother upgrading it right now. I'd wait a long while, until there is enough of an increase in technology to make your investment worthwhile.
Back to you Tom...