Dell optiplex 3040 mt

Here are my specs

16gb dd3 ram

Intel(r) core 15-6500 @320ghz

Gtx 1650 4gb

500gb Ssd

500gb hd

I want to upgrade it am I right in thinking the ram can’t be upgraded past 16gb? The cpu can only go to a i7-6700? And for a better gpu I would need to change the psu. The psu I’ve been told is hard to replace any help? 
I only want to play at 1080p or 1600-900 games like gta 5 and some newer titles. 
any help or advise would be very much appreciated