I dont have a PSU because I dont know the wattage I need so if someone can help me out that would be greatly appreciated. Here are the specs,
COOLER MASTER HAF series RC-912-KKN1 Black SECC/ ABS Plastic ATX Mid Tower Computer Case
LG 24X DVD Burner - Bare Drive Black SATA Model GH24NS95 - OEM
Seagate Barracuda ST1000DM003 1TB 7200 RPM 64MB Cache SATA 6.0Gb/s 3.5" Internal Hard Drive
GIGABYTE GA-990FXA-UD3 AM3+ AMD 990FX SATA 6Gb/s USB 3.0 ATX AMD Motherboard
G.SKILL Sniper Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1866 (PC3 14900) Desktop Memory Model F3-14900CL9D-8GBSR
AMD FX-6300 Vishera 3.5GHz (4.1GHz Turbo) Socket AM3+ 95W Six-Core Desktop Processor FD6300WMHKBOX
HIS IceQ H787Q2G2M Radeon HD 7870 GHz Edition 2GB 256-bit GDDR5 PCI Express 3.0 x16 HDCP Ready CrossFireX Support Video Card
I was thinking like a modular corsair 600W or 500W but idk.
To get your estimated wattage, go to pcpartpicker.com and choose all the parts you're using. It'll give you an estimate to plan by, but I would get a power supply (Seasonic preferably) that is at LOWEST 80 plus silver rated and at a wattage where that estimated load would be 60% of the "max" load of the PSU if you plan to overclock, and I would expect you would be with a 990FX motherboard and an FX series CPU. Corsair is nice, but only if you get one of their higher-end ones. If you want to save some money, get an XFX power supply; they're rebranded Seasonics.
Not stating your currency, it's hard for us to help suggest different parts to fit your budget, but here's one that can easily be changed.
Right now, in the link I'm about to provide, the GPU is not available in the US. But there is a 7950 for $205 I believe. Just change the drop down in the topright to US, then remove the GPU, and replace with whatever available GPU.
http://uk.pcpartpicker.com/p/1quFF