RAM could be replaced with Mushkin Blackline as an example.
GPU can be replaced with Asus HD 7850 DC2.
Both of those are perfectly valid alternatives. If anything else is out of stock, do tell and I'm sure someone'll find something worthwhile to replace it for your list.
Is there any particular reason you'd rather go for the GTX 570 though? The performance difference is quite moot imo, and the HD 7870 would probably be a better alternative at that pricepoint.