Entry level GPU

1. Which Power Supply do you have? (Be EXACT while naming the company and model)
Ans: Corsair CX430V2

2. What is your budget?
Ans: 3-4k

3. Which resolution will you game at?
Ans: 720p max

4. What are your current computer specifications?
Ans:
Processor: AMD A6 3500
Motherboard : MSI A55M-P35
RAM:Corsair Value 4GB DDR3 1333 Mhz
HDD : WD Caviar Blue 500 GB
ODD : Asus DRW-24B3ST
PSU : Corsair CX430V2
Monitor:AOC 18.5 inch LED Wide


Also, if the PSU is not enough, suggest an alternate PSU for rs 3-3.2k max
And what are crossfire and SLI? Please explain.