If the original poster could post his specs and hardware being used, I'm sure we could figure it out for him/her. I have a router running a ton of things and don't see any slow down when one of them is accessing the net heavily.
I have 1 PC & 1 Linux box ethernet wired, 2 PCs wireless, 1 iMac wireless, 1 PowerBook wireless, 1 B&W iMac (266 Mhz) wired, Xbox wireless, (2) PS2's wireless, a PocketPC I use wireless and a Tivo that connects wirelessly. All this goes through my Linksys WRT54GS router w/ Cable Modem hookup. I do have an Airport Extreme Base Station downstairs that simply acts as a repeater so the signal strength in the whole house is strong.
Like I said, post your specs, hardware and current configuration and we'll help you solve it.