What's the make / model of your PSU?
The Samsung F3 1TB is a good option
the Define Mini only has 4 expansion slots, and that board requires 5 if you want to add a second dual slot card .
and with the 2x500GB HDDs, i assume you want to use Raid?
if you are, i would advice you just to stick with a 2TB HDD for media storage and a 128GB SSD(if it fits the budget) for OS,apps and games.
2 x PCIe 2.0 x16 (x16, x8, x8 or x16, x16, x1)
1 x PCIe 2.0 x16 (x4 mode, black)
1 x PCIe 2.0 x1