X6 rendering time compared to X5

[Doug Park]

Performance is not all in the video card. If indeed you can get a faster card for your machine that may speed things up, but then you are probably going to just hit the next bottleneck, possibly processor speed or memory speed.

You can always consult the video card benchmarks to see what might be a good choice.

http://www.videocardbenchmark.net/high_end_gpus.html

How any of these cards will perform in your system is a question for someone more familiar with that hardware than I am. One could probably buy several cards and compare performance in your hardware to find the one that is best, but that seems like a pretty time consuming and expensive exercise.