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tomo-chan, it's very adorable how you are going about this.

Seriously ... looking at this budget and re-using your old case is even a consideration ..?? Yes, you might be able to use the old case ... but we'd need to know what type/size you have and which mobo you are getting.
That said, good on you for going the build-your-own. You're going to learn a lot, including how to organize a project like this bit.
That said, it will help us a lot if you provide links to the exact models you are purchasing. Knowing the manufacturer and their exact model numbers is essential to trouble-shooting and matching capabilities. For example, the RAM would be cool to know its speed and timings. The mobo ... is a ,,,
complicated component and we all (you especially included) will need to understand all of it's capabilities.
The 2 TB drives (I
hope!) are specifically intended as mass storage and you have no intention of installing the operating system on them. You should be looking to a decent SSD for your installation drive ... in fact, if your mobo supports RAID 0 for a pair of drives, a pair of SSDs in RAID will be state-of-the-art fast.
I'll trust kitamesume on the juice you need to run two of those cards, but if you stay with one card then 1000 watts should be plenty. However, there are other differences to look at, such as efficiency of the PSU, and you definitely want a modular one. Which model(s) PSU are you considering?
[
EDIT: your case, either current one or one you will buy, will need to have copious cooling capacity if you really are gonna run a pair of dual-GPU video cards. Do you care what your case looks like? Or will it be mostly hidden and you don't care (mostly)? Function and/or beauty?
Also, 128 GB of SSD should be plenty for your OS plus your apps/games. So, you
could consider getting 2 x 128 GB SSDs and RAID 0 them. Your budget is generous enough that you might even be able to swing a pair of 256 GB SSDs ... although, I barely can imagine using 100 GB of space for applications/games installation. ]