Generally speaking, Wi-Fi should have no significant impact on your ping times to counterstrike servers. The ONLY way it would is if some 2.4Ghz appliance (cordless phones, microwave ovens) happens to be interfering with your Wi-Fi connection and/or you are on the very edge of your router's range. Your neighbor's Wi-Fi can also interfere. One machine I play BF2142 on gets no better ping to the straferight.com server than my desktop with a Gigabit link all the way to my router. Your system is upstairs while the router and your dad's computer are downstairs. Try relocating the router upstairs and have your dad's system on Wi-Fi if that is possible. If it is not, try using another Wi-Fi channel or reposistion your Wi-Fi card as close to the router as possible. Try pointing your antenna downward or put it on the floor. You could also try switching to Wireless A which is hard to come by, but it uses the 5Ghz frequency range, which is much less crowded than B, G, or N. If your router happens to be one of the highly moddable Linksys WRT54G routers, you can download 3rd party firmware and double your router's signal output (Linksys ships all their routers at 50% strength). You can also purchase a Linksys WAP 54G to act as a wireless repeater to strengthen and rebroadcast the signal. Upgrading to Wireless N I don't think will do you any good because Bandwidth does not directly affect latency, and since Wireless N operates on the same frequency spectrum as G you'd be in a similar situation.
Apart from fishing a stretch of Cat5e through the wall, or along the baseboard, that's really your only good option. Home PNA networking equipment is an endangered species thanks to cheap Wi-Fi as is home powerline.