An SSD is probably the single best upgrade you can do for your computer right now. Even if you just get a smaller capacity one for the OS and commonly used programs. After using one for awhile, it's painful going back to a PC with only a hard drive.
Yup. SSDs are pretty much the best upgrade for an older computer, but just to note, you'll want to make sure that older PC can use Windows 7 or at least buy a SSD that doesn't really "need" TRIM support. That's because Windows 7 is the first Windows OS that supports TRIM.
Speaking of drives, I'd ditch the Bluray writer unless you're burning home movies or something. Multi TB external hard drives are cheap enough now if you're doing backups. BD-ROM drives are cheaper if you're just ripping / watching movies.
Yeah, I have a Blu-ray writer, and all I use it for is ripping my own movies. Although, if that is your purpose, you really need to be careful about what drive you buy, because some drives list a high read speed, but actually reduce their read speed to avoid rippers. For example, I had a Pioneer BD206 drive, and it ripped at about 2x no matter what. My LG drive can rip up to 8x or more (depending on disc position).
I've been pretty vocal about my dislike of Windows 10. I know this is a controversial opinion, but until Microsoft straightens out their privacy policy, I wouldn't pay for it. Let alone pay $100 for it. Going to be awhile before DX12 games become the norm so get Windows 7 or Windows 8.1 instead.
Windows 10 is okay. I mean... if you aren't sure if you'll like it, just get Windows 7 and upgrade. You can always fall back to Windows 7 if you end up not liking 10. However, if you do want to upgrade, you've only got that year window.
It's a bit like the debate over SLI a few years back. There's a point of diminishing returns by adding more cores. Simply because a lot of games aren't optimized for that level of multi-threading. The i5 is cheaper and the OP can always upgrade in the future if he's looking to keep costs down.
I don't think it really compares to SLI, because SLI usually does provide pretty tangible benefits. Albeit, the biggest complaint about SLI is that the scaling isn't as good as people would prefer (i.e. 100% boost with the same card added). However, they are similar in that both SLI and the i7 have had issues in the past of actually hurting performance. If your game lacks an SLI profile, you may see no benefit or slightly less performance, but if your game's threads execute on the HT cores, you may see reduced performance. Apart from that, in most games, you'll see about a 1-2 FPS increase from the extra faux cores.