HTPCs really don't require as much performance as you might be led to believe. Like daship said, a fast dual core would fit your needs fine. As for the video card, you don't need as high end of one as you may think. I would look at getting an AMD Phenom II X2 or X3 CPU and a motherboard from a good brand that goes with it. I'm not an expert on the particular models of AMD CPUs out right now, but maybe something along these lines:
+1 for not needing the i7. Thing to remember about HD, is your PC is just playing it back to you, not creating in the first place, as it would be with a high def game. The AMD should be more than enough with a half decent GPU*. Or you could treat yourself to an upgrade for your gaming pc, and use the old bits of that for the HTPC
*And only half decent. Really doesn't need to be a GTX295 or anything!
and guys..i wanna try unlocking the fourth core...and i read a lot of reviews saying that the gigabyte ga-ma790xt-ud4p is better at it... since gigabyte still gives users the option to unlock the disabled cores...
so should i go for Asus M4A78T-E or gigabyte ga-ma790xt-ud4p ??
Yes, that or the UD5P. It's a bit early for me as I won't be upgrading until the new year. I like Gigabyte boards just because I have experience with them, and they tend to overclock well, so obviously my opinion is coloured by that a bit.