This is what I did. I have a plasma screen with HDMI connectors at the back. (a lot have VGA as well). I cobbled together a computer that sits, invisibly, behind the set. I then bought this keyboard..click here and hooked it up to the computer. I connected the computer by cable to my router (you can do it wirelessly but my router happens to be near the TV). The tricky bit was to connect the DVI output from the computer to the HDMI input of the TV and a simple cable does the job which I bought off Ebay for around a tenner. You do not need to buy a 'media center' computer. If you cannot hide a desktop computer then a laptop would do.
The whole thing, using a second-hand computer and including the keyboard, should cost no more than £120. You do not need a flashy graphics card or a fast processor.