The latest generation gfx cards and mobos broadly available in the market uses PCI-E x16 interface and it's quite easy to find them and there prices are also cheaper.
OLD generation gfx cards cards and mobos uses AGP interface ( 4x and 8x ) which is obsolete now - so manufacturers are not interested too much in making them - that's why there prices are high.
HD movies can be best viewd when decoded by the modern gen gfx cards - their chipset has dedicated functionality to decode HD movies.
If you want to view HD movies with a gfx card or mobo which gfx chipset don't has HD decoding capabilities then your CPU will decode those HD movies but for that you will need a very powerful and pricey cpu but it's not very practical as even a low priced gpu like 8400GS around 1.6k is better in handling Hd movies than a powerful cpu.
Sure you could still find some old cheap agp cards based on nvidia 5xxx, 6xxx, 7xxx series in here but as per your requirement they are not suitable as those chipset don't has dedicated HD decoding capabilities - so they will offload that task to cpu and as your's is a very weak one it will be not able to decode those high resolution HD movies properly - that's why they will be very slow or even unplayable.
So to view HD movies properly you will need a gfx card which chipset has dedicated HD decoding feature like ATI HD3450 or HD4350 based on AGP interface - this cards will not overload your cpu while watching HD movies - so every HD movies will be playable in your old rig.
one caution before buying : though any AGP card should work fine make sure to check your mobo manufacturers website for any kind of compatibility issue. you have not written in your post which mobo and cpu you have.
But the only catch is their availability - it's very hard to find AGP cards based on HD 4350 or HD 3450 in the local market - so best in luck in finding them ( let me know if you find them in the local market ).
BTW, if you can then order from on-line shops
