I'd upgrade Vegas along with the build.

GPUs are replaceable over the life of the system whereas the CPU/motherboard is not generally. All these options you are listed are new and high-end, so upgrading from one to another is likely going to give diminishing returns for performance. I don't think throwing more cores at Vegas gets much more performance but check out benchmark threads here for a hint of difference.

Some Fx won't run in real time regardless of the hardware, it's important to keep expectations realistic.

@andyrpsmith @RogerS Thank you, guys! I'll upgrade to Vegas 18 as well.

@Justin-Beaton

Do you plan to run a laptop or Small Form Factor PC? The i7-10850H is hardly a workstation class CPU with a Total Design Power of 45 Watts. You didn't state what hardware you are running prior to the new build, but in your situation, I might be prone to upgrade to Vegas 17/18 instead of upgrading hardware. I got the gift of GPU hardware timeline decode with Vegas Pro 17. That was like a breath of fresh air even though I'm near the end of my hardware life cycle of four years.

Disclaimer

Feel free to disregard everything I say if you work for a corporation that has an IT department with volume purchase agreements or a green energy initiative. You won't win.

Warranty

All of my advice comes with a double your money back guarantee.

@john_dennis Haha--thanks, John! I appreciate the info. It'll be a Dell Workstation Laptop--the new Precision 7750. I will definitely be upgrading Vegas!

@Justin-Beaton

Here is a demo of the effects of hardware decode for XAVC-S-UHD.

Vegas 15 (No use of AMD UVD)

Vegas 17-452 (with AMD UVD)

@john_dennis Wow. That's really cool. I honestly had no idea. Thanks for taking the time to share that with me! So, would it be similar in both 17 and 18 or has it been improved even more in 18?

Justin run the trial first to make sure it is stable on your system as v18 is more hardware demanding - but in my view the best version yet.

@andyrpsmith Thanks, Andy. Will do.

I did a comparison like @john_dennis did, with a heavy 4K AVC file (bitrate 100,0 MB /sec) rendered in VP 16,17 and 18 with the Magix AVC UHD 2160 internet 50p template, in 16 and 17 only with the use of the Nvidia and in VP18 also with help of the Intel on the laptop from signature. These were screenshots of the renders

VP16

VP 17

VP18 with Nvidia

VP18 with Intel

@j-v Very interesting stuff. This is all super helpful! Thank you! It looks like I'll probably try to shoot for an upgrade to 18.

@j-v

It appears you didn't use (or enable) hardware decode for nVidia in Vegas Pro 17?

@john_dennis

These are my ( not default) settings in VP 1
7

and this the defaults in VP18

@j-v

I see. You use QSV decoding.

On my desktop it makes no difference, but on laptop this is for my way of working the best setting in 17. In 18 Vegas makes at the start the best settings itself on both machines.

@j-v Did the render times change? They all look to be around 2 min. with 17 a bit slower.

I don't know because that is not important for me, only rendering screenvideos for use on the forum has be quick, but therefore I don't use default templates.

In that case you're giving us an idea of relative utilization of GPU and CPU, but not efficiency in terms of system resources as we don't know how long the renders are taking.

In that case of rendering 1 minute FHD 50p for a video MP4 here, takes 27 sec. and the use of CPU and GPU's look here