List full system specs, as RIP stated, CPU is only part of what affects movie play-back. Video card, RAM would be the main ones aside from that, as well as your network speeds.
You have a dual core Intel chip, that will run HD fine. You'd only need a 3gig CPU if you were talking about a Pentium 4, and even that is not needed. I run HD fine on my T43 laptop which has a 1.8 Pentium M chip with 1.5 gig of RAM.
I don't have much knowledge with streaming but I guess the 945G video chipset is not HD capable.. Getting a discrete video card will help immensely.. Even a low end video card such as the Radeon HD 5450 will be a nice upgrade.. Your budget can easily fetch you a GTS 450 though..