If the machine you are talking about is the G5 iMac in your sig, than Yip, it is too slow,
You really need a dual G5 mac Pro min for HD,
I had a white G5 like you (128MB Graphics Card) and it would stutter on small 720P HD trailers, so for a full movie would be majorly underpowered, but It should fly on your macbook?
I upgraded to a new Alu 2.8Ghz Intel iMac last month and HD simply flies,