Ouch, your current CPU is too weak, and I doubt any of the latest mid-range GPUs might also work when paired with this processor, without creating any serious bottleneck. Though, some entry-level cards might do the job, most probably the GT series,,,,,,
CS:GO is a CPU-intensive title as well....It requires a decent processor. You would be needing a complete System Overhaul, if you plan to play all other latest games as well. Because, your current PC is very old. What's your Monitor's MODEL ?
Are you on 1080p ? Also, which power supply unit do you currently have ? Do you ONLY play CS:GO, or even other latest PC titles as well ??