T3600, best GPU before bottle-necked?

I acquired an old Precision T3600 from work and have finally decided it may be worth trying to upgrade it. It has a 635 watt PSU, 32 GB of RAM and the base Xeon E5-1603. I just bought a used E5-1660 to get the clock speed up, which will be an improvement no matter what else I do. Now I'm wondering about the GPU.

I've been eyeing the GTX 1660 Super due to its strong performance for its relatively low price (~$250). However, I'd like to get the most I can out of the GPU for a bit of future-proofing. The major limitation is going to be the CPU. The T3600 build guide says to expect some "strong" bottle-necking once you get to an RTX 2080. How much bottle-necking would be expected from the RTX 2060 Super? Will it be enough to offset any increase in performance? For example, why spend 50% more for a GPU if it's severely bottle-necked and only results in a few more frames?

While the system was free, I'm still not looking to break the bank on a PC that's seven years old. I think $400 will have to be the limit since I still need to set aside some money for an NVMe SSD. I don't know if or when I'll commit to a new build. I don't want to over-spend on a GPU now when I can't benefit from its performance, then it be out-dated by the time I do build a new system.

It's also bad timing since Ampere and RDNA 2/Big Navi are coming soon and expected to shake up prices and performance. Is it worth waiting to see what's revealed about those cards, or is it likely I won't be able to benefit from the improved performance? I'm not in a rush, and don't want to waste money on a GPU that's about to be superseded. But if they drop the prices enough on the current generation, it may be worth the wait to reap the savings, especially if I can't use the newer cards.

Thanks!