Defo get it from elsewhere as it's cheaper.
However when I upgraded the RAM in mine at the start of the year, I remember reading a lot about it and how you get better performance if you match all the RAM as the same or something.
ie:
1GB - 1GB
1GB - 1GB
1GB - 1GB
1GB - 1GB
2GB - 2GB
2GB - 2GB
Correct me if I'm wrong, but I'm sure it was something like that, which is why I went for smaller sticks, to match what I already had, rather than getting really big ones.