Overkill in the sense that the games you intend to play on it would not even cause the i7 920 to break a sweat. It's a quad-core, hyper threaded. EIGHT threads in OS.
I am assuming both have PCI slots so yes. You could buy one. Unfortunately, I'm not sure which would be best for gaming. I don't use wireless for the most part, so I'm no specialist in this category. However, I can tell you that you can't go wrong with a 300MBps Wireless-N card. I chose the Sabrent 802.11n PCI adapter
. It's pretty good in close range, but if you don't set up the antennae right for longer distances, any audio playing on your computer does a weird 'schzzt" thing about every 12 seconds. I'd suggest staying away from it. I'm not sure if what I got was a dud and has that issue, or the card does this naturally, but I'd avoid the hassle if at all possible.