Hardware needed for mp with hdtv support

Hello
I have a desktop computer with is also acting as my htpc. The specifikation:
P4 2,8 ghz northwood 800 fsb 512 mb cash
Intel 865 PERL motherboard (very stable but not possible to overclock)
2 x 256 mb standard kingston cl3 ram
ATI 9600 pro grafic card

Then i am running mp with ffdshow the computer is not fast enough to enable much filters (i have allready the picture resized to 1280x720 with is the native resolution for my projector). I would like to run mp in this resolution with the grafic card in cloned mode to the projector and in the future also to a small lcd-tv (for normal use and the projector for movies).

I am thinking about builing a new htpc, i fought about using the old cpu with a new motherboard that is stable and posible to overclock a bit, for example asus p4c800 series. How will this be together with maybe more memory/faster grafic card compared to a new amd 939 nforce 4/intel 775 socket motherboard cpu with stuffs like sli etc? I know its a hard question to give a good answer to, but what i want to knew is that is stopping the computer now - lack of cpu, memory, the grafic card or something else. If memory is the problem - do i need more memory, faster memory or both? Would a better grafic card do any difference? The htpc will only be used for mp and ffdshow, no gaming. For tv i am currently using a leadtek analog tvcard with integrated mpeg decoder.

My idee is to use the cpu and maybe the memory and the graficcard on a new motherboard and get a cheap celeron for the desktop computer. Of course price is a matter, but if i get a lot better setup with a newer platform i will wait.

I have searched a lot in different forums but the more i surch the more confused i get............ So please help me to decide what i need for a decent setup.

Sorry for my bad english!
/O