R9 295X is not ideal because it uses a hybrid cooler and has a very low Powertune max target temperature, which causes it to aggressively throttle. Dual GPU cards are almost never the way to go, but the R9 295X is just the epitome of bad dual-GPU cards. It is most certainly better to go with 2x R9 290X instead of one R9 295X.
GTX 980 has the best Perf/$ ratio on the market today. It is comparable to the R9 280X in single hash brute force performance, but faster than the R9 290X in multi-hash performance and other attack modes such as combinator attacks, at about half the power and heat of the R9 290X.
Building a password cracking cluster is not near as simple as "throw some GPUs into a chassis and start cracking passwords." There are a lot of design considerations, especially with heat and power concerns. Six nodes with four GPUs each will draw a lot of power and generate a ton of heat. For a 24-GPU cluster you really need to use special hardware that is built for this purpose, otherwise you will likely experience a ton of failures and headaches.
Also, why do you want to use Passware?
GTX 980 has the best Perf/$ ratio on the market today. It is comparable to the R9 280X in single hash brute force performance, but faster than the R9 290X in multi-hash performance and other attack modes such as combinator attacks, at about half the power and heat of the R9 290X.
Building a password cracking cluster is not near as simple as "throw some GPUs into a chassis and start cracking passwords." There are a lot of design considerations, especially with heat and power concerns. Six nodes with four GPUs each will draw a lot of power and generate a ton of heat. For a 24-GPU cluster you really need to use special hardware that is built for this purpose, otherwise you will likely experience a ton of failures and headaches.
Also, why do you want to use Passware?