Bottleneck:
The one part in your setup that reaches it's limit first.
Example: You have a super cpu that can crunch 100 GB/s of data. But you are reading that data from your hdd, which can only provide 30MB/s. Then this is your bottleneck for the application.
Important to note, that this changes with the use case. Encoding movies has a different bottleneck than playing FarCry.
And I agree with Random32 on this. If the application is gaming, then the graphics card is your bottleneck. This means you can downgrade your other hardware a lot, it will still be the GPU that dictates how much of a gaming machine you have.
If you want gaming, pick the best GPU you can afford, then build the rest of your system around that.
HDD prices:
I bought 8 TB of anime storage for under 200€ last year. The current prices are still really high compared to that. And then you'd usually expect last years hardware to go down in price not up.
But this doesn't really matter if you only want a 1TB hdd. Not with the budget your are using here. But for really building up storage, this is still a bad time.
Oh and I have used Seagate drives for 10+ years now (that 8TB too) and never had one fail on me. In fact the ones from 10 years ago still spin up fine. Statistics vs anecdotes...
