Should I Upgrade or Build a New System?

[cityracer]

My current build does not suit my needs. It regularly gets to 75*C whenever I play a fairly demanding game. It also lags when I play games like Europa Universalis 4 with big mods.
My current build is this:

CPU: AMD FX-6300 6-Core Processor Black Edition
GPU: HIS 7870 IceQ 2 GB(256bit) GDDR5 2x Mini-DiplayPort HDMI DL DVI-I (HDCP) PCI Express X 16 3.0 Graphics Card H787Q2G2M
Motherboard: ASUS M5A78L-M LX PLUS AM3+ AMD 760G Micro ATX AMD
Hard drive: Western Digital Caviar Blue 500 GB SATA III 7200 RPM 16 MB Cache Bulk/OEM Desktop Hard Drive - WD5000AAKX
Optical Drive: Samsung Optical Drive SH-224DB/BEBE
Ram: Corsair Vengeance 8GB (1x8GB) DDR3 1600 MHz (PC3 12800) Desktop Memory (CMZ8GX3M1A1600C10)
Power supply: Antec VP-450 450 Watt Energy Star Certified Power Supply
Case: Thermaltake V4 Black Edition VM30001W2Z No PS Mid Tower Case (Black)
OS: Windows 7

I know that I would replace the case with a Apevia X-Hermes Metal Case with Side Window-Green X-HERMES-GN, but I don't know what else I would upgrade. I also wouldn't know where to even start selling the old parts.

If I build a new one my part list would be this:

CPU: Intel Core i5-4690K Processor 3.5 GHz LGA 1150 BX80646I54690K
CPU Cooler: Cooler Master Hyper 212 EVO - CPU Cooler with 120mm PWM Fan (RR-212E-20PK-R2)
Motherboard: MSI ATX DDR3 2400 LGA 1150 Motherboards Z97 PC MATE
GPU: Gigabyte GeForce GTX 970 Overclocked GDDR5 Pcie Video Graphics Card, 4GB
RAM: Crucial Ballistix Sport 8GB Kit (4GBx2) DDR3 1600 (PC3-12800) 240-Pin UDIMM Memory BLS2KIT4G3D1609DS1S00/BLS2CP4G3D1609DS1S00
Hard Drive: WD Blue 1TB SATA 6Gb/s 7200rpm Internal Hard Drive
Case: Apevia X-Hermes Metal Case with Side Window-Green X-HERMES-GN
Power supply: XFX TS 550w Full Wired 80+ Bronze Power Supply - P1550SXXB9
Optical Drive: Samsung SATA 1.5 Gb-s Optical Drive, Black SH-224DB/BEBE
OS: Windows 7
Total cost: $1034.08

I already know I can get $500 from selling my current PC. If I were to Upgrade, I'd want to pay <$400, because after that it would be dumb not to build a new one. Also, whether I upgrade or build a new one, It will cost an additional $60 to get it put together.

What would upgrades would be the best, and is it worth it to upgrade?

 

[logainofhades]

I would start with the better PSU and Cooler, and try that first. You are thermal throttling, badly, with that kind of CPU temp.

 

[cityracer]

I would start with the better PSU and Cooler, and try that first. You are thermal throttling, badly, with that kind of CPU temp.

I'm not good enough at computers to know what to look for. Unless I'm told exactly what to get, I'm completely lost.

 

[logainofhades]

This cooler is a good one, and will fit your current case. The power supply is more than you currently need, but will give you some upgrade headroom. I would have went with an XFX 650w, but this Evga was only $10 more and is modular.

PCPartPicker part list / Price breakdown by merchant

CPU Cooler: CRYORIG H7 49.0 CFM CPU Cooler ($34.50 @ Newegg)
Power Supply: EVGA 750W 80+ Bronze Certified Semi-Modular ATX Power Supply ($79.99 @ NCIX US)
Total: $114.49
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-04-30 09:59 EDT-0400