Browsing = Wont affect ping or game performance
MSN = Wont affect ping or games performance
Downloads = Depending on download speed will have varying affects on your pings.
It may be worth considdering a wireless network for your needs. It elliminates the hassel of dragging wires through walls or around doors and means you can extend te network with ease if you add another computer later.
You can hard wire your computer to the wireless router so dont worry about games performance, and you can have your sisters connected over the wireless.
Wireless Router
PCI Wireless card
USB wireless NIC The usb may be the best option for you as you can switch it between PC's if you need to (Handy if you have a friend with a laptop you want to do multiplayer games with).
Either way this wont cost you more than £45, not bad considdering you have removed the hassel of the wiring
