Go with Sempron setup if you are gonna do more downloads. If more movie watching especially HD ones then i would suggest this
Processor: AMD AthlonII X2-240 - 3k/Processor: AMD Sempron 140 - 1.8k
Motherboard: Biostar A785GE - 3.3k(HD 4200 is a very good IGP. Plays HD with eaze and directX 10.1 complaint)
RAM: Transcend 2GB DDR2 800MHz - 2.1k/Transcend 1GB DDR2 800 MHz - 1.1k
HDD: Seagate 500GB SATAII 7200.12 series- 2.3k(Avoid he 7200.11 series. Lots of firmware issues)
This comes to 8.5k/9.5k/10.7k. I would sugest you go for the AthlonII X2 240+1GB RAM set up as it has a dual core CPU which comes in handy while watching HD clips. Plus you dont need a 2GB most of the times unless you are using a ultimate version of vista.