My PC is 7 yrs old. RAM is 500Mb.

Is this sufficient to stream TV?

I suspect not.

How much would you recommend and what would it cost approx?

It's not so much the RAM that you'll need as a fast processor and an adequate internet connection.

However, if you want to upgrade the RAM I would have thought that 4GB would be fine - that's the sort of minimum level that most laptops are supplied with nowadays.

You might be able to upgrade your computer's RAM, or it might already be operating at the limit of it's capacity. If you can it's not very expensive - certainly less than £100. This website http://www.crucial.com/uk/ will scan your computer and tell you what's what (look for the "System Scanner").

I also wrote about an alternative to using your computer: Click here