I feel like my GPU is the bottleneck of my system, or at least I feel it should be, given its age. I'm looking to upgrade to something around the 100 euro range.
Current specs:
Intel Core i5-2500k CPU 3.3GHz
8 GB RAM (I believe DDR3, can check if important)
ASUS P8Z77-VLX motherboard
Win7
A radeon 7850 or geforce 650Ti should be available for just over 100€, that's the best you're gonna get for that kind of money. Both will be a huge upgrade.