The best 3.5" hard drive you can buy at the moment IMO for SFF pc's and A1200/A600's is......
Samsung Spin Point 40GB
It's very slim, light and gives out little heat as the 40GB model uses only one side of a platter.
In other words, its low profile would fit into an A1200/A600 like a dream if you have the correct cables!
Also, the thing has near-inaudible seeks (quietest I've EVER heard) and very low operational sound.