You'll need a router that's wireless capable. The Wii is wireless anyway so that won't be a problem. To make the 360 wireless, you'll need to buy Microsoft's Wireless Adaptor that's around the �50 mark, and if your PC is not wireless compatible, then you'll need a wireless dongle for that at around �20.
If you want to save a few quid, then I'd suggest changing your ISP. The new one will happily chuck you a free wireless router, this just hooks up to your phone line in the same way as your modem and then you can either connect things with cables to the router or run everything wirelessly. If you don't want to change your ISP, then try ringing them and tell them you want to leave as XYZ ISP will give you a free wireless router if you join them and you really need to go wireless - they may well give you one for free to keep you.