If you want to stay with the older AMD machine you have, you're going to have to go with a 'better' video card to minimize the amount of CPU that is required to run video.
The card is going to have to have on-board HD decode and preferably HDMI with integrated audio decoding too. The more oomph you put into a video card the easier the rest of your system will be.
AMD did make a Socket A Sempron 3000 that is really a full-blown Athlon MP. It got the Sempron name because at the time, AMD was making the big shift to 64 bit / multi-core and wanted the Athlon name to move forward with that tech.
Anyway, A Sempron 3000 should slot right into your machine. That and another gig of ram will make quite a difference.
But do add up the numbers. Say a new cpu and ram costs you $150 and a video card another $100-.
You still have a 5+ year old machine.
A new mobo/ram/dual-core cpu comb0 deal with modern on-board video can be found as package deals for approx $300-.