Kudos to you on actually caring about system requirements
GDDR5 is inherently twice as fast as DDR3 just by nature of its "quad pumped" design, ie you multiply the base clock of GDDR5 by 4 to get the effective clock, and multiply DDR3 by only 2. DDR3 proves to be a bottleneck in most cards simply because of its lower bandwidth, but the fact that there's double the amount doesn't matter much. Essentially, more RAM is good for gaming at higher resolution with better quality textures, but the problem is the processing power of the GPU itself; the core will run out of steam long before it runs out of RAM. Since you're trying to find system requirements, I actually think the R7 240 would be more useful. This would mimic the many, many integrated graphics cores out there that use system DDR3 as their source of memory, and would give you a more realistic picture of what users, especially those on laptops, have. Most IGPs these days can utilize ~1.7GB of system RAM if necessary, so you can use something like MSI Afterburner to monitor VRAM usage while you're testing and make sure you establish a realistic target, whether it be 1, 1.2, 1.5, 2GB, etc.