What components should i go for if i want to upgrade my current PC for gaming?

[Crystal0015]

Hello.
First of m new to this whole stuff and really in for some cool stuff
So, after a bit research i came to know that my current config only has one thing that is good enough as per my budget. And yeah my budget is very tight.
The only good thing that i have is my mobo i.e. m5a78l-m lx v2 (correct me on this one if it isnt for mid-tier or medium gaming pc.)
Suggest me good CPU,GPU and other things required for converting it to somewhat a "gaming" pc. My budget is (for CPU+GPU+other stuff) : 300-340$
Thanks alot
Edit #1: For info on my PSU its a 450W one. dont really think it would handle good stuff(cpu or gpu) since its brand is UMAX (some china one i guess).

 

[LucoTF]

What re your full current specs?

 

[Crystal0015]

What re your full current specs?

CPU : AMD Sempron 145 @2.80 GHz
MOBO : ASUS m5a78l-m lx v2
PSU : Zebronic 450 Economy Series (DSATA) 20+4 pin
RAM : 2GB @ 800MHz
HDD : 500GB
I dont have any GPU as of now.
if you need more info ask me

 

[Elemential]

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Pentium G3258 3.2GHz Dual-Core Processor ($62.99 @ NCIX US)
Motherboard: ASRock H81M-HDS Micro ATX LGA1150 Motherboard ($49.99 @ Micro Center)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory ($59.99 @ Amazon)
Video Card: Asus Radeon R9 270X 2GB DirectCU II Video Card ($165.99 @ SuperBiiz)
Total: $338.96
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-03-21 06:10 EDT-0400

Best upgrades you can make

If you want to go amd:
PCPartPicker part list / Price breakdown by merchant

CPU: AMD Athlon X4 860K 3.7GHz Quad-Core Processor ($78.99 @ NCIX US)
Motherboard: ASRock FM2A88M-HD+ Micro ATX FM2+ Motherboard ($54.49 @ Newegg)
Memory: G.Skill Ripjaws Series 8GB (2 x 4GB) DDR3-1600 Memory ($54.99 @ Newegg)
Video Card: EVGA GeForce GTX 750 Ti 2GB Superclocked Video Card ($124.99 @ NCIX US)
Total: $313.46
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-03-21 06:16 EDT-0400

 

[Crystal0015]

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Pentium G3258 3.2GHz Dual-Core Processor ($62.99 @ NCIX US)
Motherboard: ASRock H81M-HDS Micro ATX LGA1150 Motherboard ($49.99 @ Micro Center)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory ($59.99 @ Amazon)
Video Card: Asus Radeon R9 270X 2GB DirectCU II Video Card ($165.99 @ SuperBiiz)
Total: $338.96
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-03-21 06:10 EDT-0400

Best upgrades you can make

If you want to go amd:
PCPartPicker part list / Price breakdown by merchant

CPU: AMD Athlon X4 860K 3.7GHz Quad-Core Processor ($78.99 @ NCIX US)
Motherboard: ASRock FM2A88M-HD+ Micro ATX FM2+ Motherboard ($54.49 @ Newegg)
Memory: G.Skill Ripjaws Series 8GB (2 x 4GB) DDR3-1600 Memory ($54.99 @ Newegg)
Video Card: EVGA GeForce GTX 750 Ti 2GB Superclocked Video Card ($124.99 @ NCIX US)
Total: $313.46
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-03-21 06:16 EDT-0400

Thanks BUT Where is the PSU? i mean all this will work with the current psu?
If current one wont work then please suggest me something that is compatible with my motherboard.
What i mean to say is i dont want to change my motherboard if it is possible (you can surely suggest me other if its in the budget) and upgrade my current specs. (including psu,gpu cpu etc or everything essential..)

 

[LucoTF]

Cpu: fx-43xx or fx-63xx
Ram: 8gb ddr3
Gpu: if keeping the psu then gtx 750 or 750ti
If upgrading psu then r9 270

Thats as good as you will get on that budget