I would definately not recommend mixing RAM of different sizes, speeds, or brands.
Ideally, you will want to get a matched pair. Some good brands to look at are OCZ, Kingston (Hyper X Series), Corsair, G Skill, or Geil. You should be able to pick up a matched pair of 1Gb sticks (2Gb total) for around $100AUD ($40-$50 USD).
~Sol