Personally, I feel Nvidia has slightly better drivers than AMD. (Feel free to contest this as drivers are always a case of hit and miss.) Buying GPUs can be tricky thanks to individual manufacturers too.
It depends a bit on your resolution. I think any midrange card of today should be well suited to good quality gaming at 1920x1080 in the years to come. (I ignore all the super high resolution/4K hoopla.)
Vram plays a factor as well. For example, my 3 year old 260 GTX core 216 would stutter with most high res packs at Full-HD as it only has 896 MB Vram. Fine tune it with optimised packs and it can still be silky smooth. For comfy Full-HD gaming with high res packs I would recommend at least 1.5 GB Vram. (I monitor resources with GPU-Z.)
Of course, the game and CPU used will dictate other factors. Almost all current games take advantage of 4 cores so it should be a minimum for future-proofing.
Some like Crysis 1 love super fast clock speeds over core count, others like Max Payne 3 are as happy with a (relatively slow 2.66 GHz) 1st gen i7-920 as a current gen i7-3770k.
Strategy games like Civ V can love hexacores too.
Some like Skyrim and GTA 4 prefer 4 cores and fast speeds. (Especially the latter.)
Not all games take advantage of HT as well so i5s are solid choices on a tighter budget.
Long story short, way too many factors play a part. Check out review sites like Guru3D, MaximumPC and tomshardware.
(Wonders if Intel hexacores will ever reach the mainstream socket, also waiting for Nvidia's 660 ti.)