I’ve had quite the experience shopping around for cards, testing hashrates, voltage settings, etc. and have developed a keen understanding of mining GPU’s through extensive programming experience.
The most stable cards for mining are NVIDIA because they never crash. AMD’s cards are reliable and allow for customization due to removed safety parameters. Simplemining offers OC options for NVIDIA that are user friendly and recommended for beginners.
Claymore's dual-miner software allows for a 13 GPU system with 13 different customizable cards. Theoretically, if you kept finding different cards you could keep mixing and matching; or even adding them all if you have enough ports.
Top performing AMD cards are the 4 series and 5 series. Some cards offer just under double the hashrate, but are more than triple the cost. Some mining sites (like Miningstore.co) have released AMD GPU’s with OC settings programmed into the firmware of the cards. Avoid any manual hacking to avoid killing your graphics card and shop through reliable retailers like Miningstore.co for the warranty value. These OC settings are not available on NVIDIA cards due to the aforementioned safety parameters, but AMD’s more open point of view allows for a nearly 30% increase on hashrate and 10%-20% voltage decrease (power cost reduction) on the 4 and 5 series AMD cards (great for miners and gamers). These settings can also be built into your card to avoid requiring programming from Afterburner and Trixx. Saving power cost and increasing the capabilities of your card is a win win, and the 4 and 5 series AMD cards do it best.
Knock this out quickly as possible as large-scale mining operations are buying up every card they can find. Newegg, Amazon, eBay, Intrex, TigerDirect, Best Buy sell out rapidly upon receiving new shipments. The best way to purchase if you want to OC them through the firmware is to purchase through major retailers with pre-installed settings. Minigstore.co is the top retiler for this purpose. Be sure to check out formums for GPU mods to get this done free, but be careful risking to kill your card. Some users charge a few hundred dollars just for the firmware settings not including the card.
All in all, at the current mining scene, AMD 5 series cards, especially 8GB RAM cards are the way to go. Namely, the Sapphire Nitro+ RX 580 8GB and MSI TWINFROZR RX 580 8GB are reliable, powerful, and easy to tweak, making them the best option for mining coins.