Honestly from those specs and your request. in order for that to play HD it will cost a pretty penny. It would be wiser to just get a new computer. your can find reasonable new computers that can do what you want for 250-400
Your system is out-dated... you are better off buying yourself a new cheap computer.
You could try upgrading the RAM to 1GB (or higher) this will help you run more tabs in browsers and more programs but I dont think your processor will cope with it.