APUs (your A8) are for general PC users for low end, low power, low demands, that are /not/ made for gaming and will choke any optional GPUs you put in. Any Gaming GPU demands high power, a off the shelf PC has low power PSU, so in addition to the over $300 for a decent PC Gaming GPU you need to include the price of a PSU to power it.
In honesty you need to spend $700+ for a PC to play "new to this beautiful world of PC gaming" as that is what it costs for the hardware to make it work like that. PC Gaming is expensive if you can't afford to replace your system (you need to replace CPU, the GPU, and that means PSU as well) then stick with a PS4, for only $200-400 you can play GTAV 1080P High @30FPS and many other titles at 60FPS plus you get free BluRayPlayer (3D!!), FREE Wireless Controller, FREE Gamer's Headset that is $200 alone you have to add on to any upgrade costs you would be considering when replacing your PC as well.
Care to check for yourself? Simply go to www.canIrunit.com and select the game and it will tell you how your computer stacks up.
REMEMBER: MINIMUM Means playing 1024x768 Low graphics and getting AT LEAST 15FPS
RECOMMENDED: Is the true 'gamer's experiance' as 'I saw on youtube'.