Vista - Better cpu or better graphics needed?

Hi everyone, this is my first post, i hope someone can help me with my problem. I have recently started to download .mkv video files to watch, i have tried several different players and codec packs but none of which will play several of the files smoothly. My cpu power meter is at 100% also which i imagine there is more power required to play the movie files. Although i also wondered if it was the lack of a decent graphics card as mine is only 128mb. I was looking into buying one of the GeForce 1GB models, either the 9500 or the 9800, would this be a waste of money as it may be my actuall cpu that is letting me down?

I have a dell dimension 5150 with an intell P4 @ 2.66 GHz with 3GB of RAM

it is prolly a combination of both..... how much would you like to spend on upgrades?

Well i was looking at cards on play.com and was going to go for something up too about £60-£70, i dont really want to go further with upgrading the cpu to be honest as im happy with it the way it is, i only thought the graphics card upgrade would be a good idea as it was only 128mb.

i suggest to buy a graphic card,cheap 256ram and then a better cpu. duo core. it should add up. but it worth it. also why do u have so much ram.
wha is importatnt is

ram,cpu, graphic card

I bought the pc from a friend that was using it for music purpose's, so the cpu and the ram etc was what was in the machine when he sold it to me. Why would i buy more ram, wouldnt 3GB be sufficent? If its going to cost more of an out set of say £70 all in then i wont really bother with the added card upgrade or cpu, it was only so that i could play .mkv files smoothly anyway as i dont really play games on the pc. I tried copying the .mkv files to my ps3 but it says even small files such as 200mb mkv files are 94GB for some reason, i take it the ps3 doesnt support every aspect of the mkv files.

you can get a cheap processor and gpu all for under....... 100 i think depending on what you get

u can.

512MB Pcie nVidia Geforce 9400GT Video Graphics Card GF - eBay (item 250362347885 end time Feb-22-09 16:01:00 PST)

and cpu o fur choice

i would not buy a used gpu or cpu because even if it does work you never know what its been through with the last owner
so try to get a new one

hmm, im a little unsure what to do, as i say, i imagined a better graphics card would do the trick, but with the added expense of a new cpu on top, its maybe not worth it just to watch some video files of HD content.

well just a new gpu will help but not as much as both would