[Build Help] Best cost/performance from 7870

My current build is this:

PCPartPicker part list / Price breakdown by merchant

Type	Item
CPU	Intel Core i5-4460 3.2GHz Quad-Core Processor
Motherboard	ASRock H97M Micro ATX LGA1150 Motherboard
Memory	Team Vulcan 8GB (2 x 4GB) DDR3-1600 Memory
Storage	Crucial MX100 128GB 2.5" Solid State Drive
Storage	Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive
Video Card	MSI Radeon HD 7870 GHz Edition 2GB Video Card
Power Supply	Corsair Builder 600W 80+ Bronze Certified ATX Power Supply
Operating System	Microsoft Windows 10 Home OEM (64-bit)
Case	HP ATX 17.5x39.4x40.9 cm Black
TV Monitor	Dynex 720p 1360x768 HD Television
Audio	Logitech G230 Stereo Gaming Headset
Keyboard	Cooler Master Devastator
Mouse	ZPS 3200 DPI 7 Button LED Optical USB Wired Gaming Mouse
Controller	Microsoft Xbox 360 Controller
Optic Drive	DVD Drive
	Prices include shipping, taxes, rebates, and discounts
	Total (before mail-in rebates)
	Mail-in rebates
	Total
	Generated by PCPartPicker 2015-11-23 21:08 EST-0500

I am currently deciding between the MSI Radeon R9 380 and the XFX Radeon R9 390.

Everytime I look for comparisons, it's only ever a Crossfired R9 380 vs a R9 390x and seems to think there's no way I'd use one card or the 390 without the X. Obviously, the 390 is better, but I want to know if it's good enough to warrant the 90 more dollars in cost (before rebates).

Main concern is if my rig can use the 390 without wattage issues as I've been told both that it's fine and that it's too low. Secondly is that it's worth the update to a 380 4gb from my 2gb 7870 for 200 and if not how much better the 90 dollars is getting me for the 8gb 390.

My current tv monitor is only 720p, and I want a 1080p or better monitor, but I'm sure I'm needing more strength if I bump that up. Is it worth it to get the 1080p monitor first or the gpu then the monitor? I can probably only afford to get one or the other during the black friday deals.

Should I even bother upgrading or is my rig at the point of just saving up and replacing the entire thing?

Submitted by LycaNinja Xbox 720 Release Date 2015