CPU
Intel
Pentium G3258
i3 4xxx
i5 4xxx
Xeon E3 123x V3 ( like a locked i7 without the integrated graphics)
i7 4xxx
--
AMD
FX 6300
FX 8320/E
Motherboard
Intel
H81
H87
Z87 with sli/crossfire and overclocking capability
H97
Z97 with sli/crossfire and overclocking capability
--
AMD
overclocking capability comes standard
970 (with possible sli/crossfire capability)
990FX (sli/crossfire standard)
RAM
1 x 4 GB
2 x 4 GB
2 x 8 GB
Storage
1 Tb, 7200rpm, 64Mb cache
120+Gb SSD, 1 Tb, 7200rpm, 64Mb cache
240+Gb SSD 1 Tb, 7200rpm, 64Mb cache
GPU
iGPU
R7 250x
R7 265
GTX 750ti (no PCIe needed)
R9 270/x
GTX 760
R9 280
R9 280x
GTX 770
R9 290
GTX 970
PSU
300-450W
500-650W
750W and above with four 6/6+2 PCI cables ( required for sli/crossfire)
Case
ATX Full Tower
ATX Mid Tower
MicroATX Mid Tower
MicroATX Mini Tower
Extras
aftermarket cooler
case fans
fan controller
wireless network adapter
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That's what I have so far. The list can be further broken down by price where applicable.