Which card should I use(I plan to add another one in the future). Sapphire R9 290X Tri-X 4GB, or Sapphire R9 290 Tri-X 4GB. What are their cons and pros?
As BigChees3 has said, the 290 and the 970 are of similar graphics capability, with the x being overclocked and a little better. The 290x uses as much power as two 970s, so you need a significantly bigger and more expensive power supply, and you will be dumping a lot more thermal energy into your case and/or the world, so cooling will need to be improved. If you plan to use two GPUs, I would look very hard at the GTX 970.
The 290 can be significantly cheaper than the 970, but the 290x is about the same price.
Here's a simple comparison answer to your original question.
x is just the overclocked version, eats a bit more power. but why not nvidia? that card needs a big psu, and if you wanna xfire it it needs even a bigger one, instead of spending 300 for the card and the required psu to run it take a gtx 970.
As BigChees3 has said, the 290 and the 970 are of similar graphics capability, with the x being overclocked and a little better. The 290x uses as much power as two 970s, so you need a significantly bigger and more expensive power supply, and you will be dumping a lot more thermal energy into your case and/or the world, so cooling will need to be improved. If you plan to use two GPUs, I would look very hard at the GTX 970.
The 290 can be significantly cheaper than the 970, but the 290x is about the same price.
Here's a simple comparison answer to your original question.