Peachesbackwards

Would appreciate anyones help & advice here because though I'm fairly computer savy, I am totally unclear what my minimum requirements should be for a laptop or notebook which I'll only be using for the internet via my High Speed Roadrunner Service (right now up to 5000 download & around 375 upload) including email. I have an excellent HP desktop Media computer in my second bedroom but as I'm lazy I would like to be able to be on the web when lying down in bed instead of having to get up and go to my other room with the desktop. I would not be using the laptop for anything else. I'd be using a wifi fast speed cable connection as I said through Roadrunner. I don't want to have a desktop in my bedroom for cosmetic reasons.

My desktop computer is 120 Gigs, pentium4, 512 RAM, 900 MHZ, XP & I'm connected on the web using Firefox via Roadrunner High Speed Cable. I'm trying to buy the cheapest used working laptop or notebook on Ebay that I'll be able to surf the web as the same speed as my above desktop computer & won't be downloading any other software programs on it except for Virus & ANtiSpyware programs.

What is the minimum operating system I need for a laptop/notebook just to run at the same speed & efficiency on the web as my desktop in terms of gigs, pentium, ram, mhz & operating system. Any help welcome & appreciated.