I am planning on upgrading my CPU to the i7-3770.

Are there any utilities that can monitor bottleneck status ... Like core-temp monitors cpu/gpu temperature?

Bottleneck calculation surely should be added to HWINFO. Has anyone asked for this to be added?

The best way to get past the I5-2400, if that's what you want to do, there's many good deals on refurb 7010/7020/9010/9020 MT's.  Then PSU's can be upgraded too.  Still somewhat limited by the CPU, but still more options.  The 3010/3020 I skip on purpose since they're more limited than the aforementioned.  Up to you what you want to do.

If you let us know what CPU you currently have, not too many options with an SFF, but a GPU can certainly be recommended.

Good question @donm4599x.  Win10 already has some gaming things built in, like being able to check FPS, screen record, etc.  And so why not bottleneck?

I did run across a bottleneck YouTube video. 

Don't be fooled just by socket types though - the 390 can't take 3rd gen.  While the I5-2400 is the top listed CPU in the tech guide, I don't know if it can take an I7 2nd gen.

Optiplex 390 Tech Guide 

Granted one can be aware of it, best to check bottleneck beforehand.

I still don't know what CPU you currently have.  So, for example, if you went from an I5-2400 to an I7-2600, it would give you 28% better performance according to game-debate.  For daily tasks, you probably wouldn't notice the difference.  Performance has to be almost double to notice a difference in speed.

However, here's what game-debate had to say about gaming:  Jan 9, 2011 · In terms of overall gaming performance, the Intel Core i7-2600 4-Core 3.40GHz is massively better than the Intel Core i5-2400 3.1GHz when it comes to running the latest games. This also means it will be less likely to bottleneck more powerful GPUs, allowing them to achieve more of their gaming performance potential.

I found a couple of 390's being sold on ebay with the I7-2600:

https://www.ebay.com/itm/Dell-Optiplex-390-MT-PC-Core-i7-3-40GHz-8GB-RAM-128GB-SSD-2TB-WIFI-Win10-Pro/233586861086?hash=item3662dca41e:g:FlUAAOSw~zdevDH3 

https://www.ebay.com/itm/Dell-Optiplex-390MT-Core-i7-2600-Windows-10-Pro-Desktop-PC-500GB-HDD-8GB-RAM/363001568064?hash=item5484945b40:g:qiIAAOSwmwVeyIad 

So if you try it, make sure you have the latest BIOS for your 390 first.

The best combo I could find:

Graphic card and processor will work great together:

Intel Core i5-2400 (Clock speed at 90%) with NVIDIA GeForce GTX 1050 Ti (Clock speed at 90%) x1 will produce only 1.16% of bottleneck.

In contrast:

Graphic card is too weak for this processor:

Intel Core i7-2600 (Clock speed at 100%) with NVIDIA GeForce GTX 1050 Ti (Clock speed at 100%) x1 will produce 100% of bottleneck. Everything over 10% is considered as bottleneck.

I could not immediately a lower watt GPU to go with the I7-2600 without bottleneck issues.

Graphic card is too weak for this processor:

Intel Core i7-2600 (Clock speed at 100%) with NVIDIA GeForce GTX 1650 (Clock speed at 100%) x1 will produce 15.62% of bottleneck. Everything over 10% is considered as bottleneck.

When I checked the 1650 against the I5-2400, then the CPU was considered too weak.

Optiplex SFF's + GTX 1050 ti + YouTube List 

Sometimes only a 30w card might get recommended, but some YouTubers have been running 75w cards on 240w PSU's for some time now.

So, if you have an I5-2400, it might be your best bet to keep using it.

You can always do your own searches if you want to - like if you want to find a 75w or less GPU to go with the I7-2600.