$400 USD or $400 AUD? This is what you can get with $400 USD. I'd recommend 8GB RAM, but it doesn't fit the budget. Just buy another 4GB stick in the future.
You must login or register to view this content. /
You must login or register to view this content.
CPU: You must login or register to view this content. ($103.99 @ SuperBiiz)
Motherboard: You must login or register to view this content. ($50.98 @ Newegg)
Memory: You must login or register to view this content. ($24.99 @ Newegg)
Storage: You must login or register to view this content. ($52.49 @ OutletPC)
Video Card: You must login or register to view this content. ($97.73 @ Amazon)
Case: You must login or register to view this content. ($34.99 @ Micro Center)
Power Supply: You must login or register to view this content. ($42.99 @ SuperBiiz)
Total: $408.16
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-05-05 15:18 EDT-0400
Don't go with an AMD APU. They're pretty much obsolete if you're trying to buy brand new parts. I would only recommend an APU build if it's used and cheaper than $300. A decent APU build with all brand new parts will run you around ~$350. For ~$50 more, you can get a dedicated GPU, such as the R7 260X, which will outright destroy any APU build.
I would recommend at least $500 for a decent gaming build.