The general rule for APU RAM is 'faster is better'. You're asking your RAM to take on the role of graphics RAM, so make it as fast as you're willing to spend (but within the realms of what your mobo supports).
Also factor in that you'll have to allocate some of that RAM in the BIOS, so if you allocate 2GB, you're left with 6GB (ish) for system RAM. For what it's worth, I run an A8-7600 with 8GB (2GB allocated), and it's fine for low-demand 1080 stuff. Most games since 2015 have to be on low-med settings, depending on their graphics demand. However, I don't play much of the recent stuff, so I don't notice it so much.
The Vega 11 graphics will be around half as good as a GTX 1050, or maybe similar to a GT 1030 (in benchmark terms), but the RAM speed can help. However, the gains might be single figures of FPS, which isn't great, but if you're taking a high 20s FPS into the low 30s, it's down to whether the added cost is worth it to you.
Consider the 2666 vs 3200 in this vid -
https://youtu.be/3D5ahmvCceM
Personally, I'd get the RAM I plan to have for the setup with the 'proper GPU', and make the most of it, regardless.