Re: white pixels, take a look at this thread:
The suggested fix has mostly worked for many people.
Slowdowns and particle effects disappearing can happen if your system is bogged down. In the later game stages and on big maps with lots of players it gets very memory intensive. Part of your problem is running Vista with only 2gb of memory. The OS itself eats a big chunk, leaving less for the game. You could try dropping your texture quality settings a little bit, as the textures is the main memory eater so it should go a bit smoother.
Also make sure you're running the latest patches, 1.16 for Sins and 1.02 for Entrenchment.
Edit: I realized the white pixel solution I posted for was Nvidia cards, but I'm sure ATI has something similar. I don't remember any ATI-specific solution posted on the boards.