Although the Mystique 632 is dated it's not a bad case.
*If 140mm fans will fit (you will need 140x140mm for fans to fit enlarging the vent holes to 135-140mm diameter circles and using the existing 120mm mounting holes would do wonders to airflow. If you can't fit 140x140mm than 120mm fans will work. Reason I suggest 140mm fans is because they move 50% more air than 120mm fans; 2x 140mm fans move as much air as 3x 120mm fans of same design.
*If you only need one 5.25 optical bay you can mount another intake fan in the other 3 bays (usually 140mm is about 3mm too big).
*Having PSU in top means you can do a 140mm intake directly below where GPU mounts to cool it. If you do this I suggest raising your case so you have 40-50mm bottom clearance for easy airflow to bottom fan. There are a couple of companies who sell magnetic mounting filters; Silverstone and DEMCiflex and they both work well. I use casters on a custom base with openings to match vents. Makes it easy to move case as needed too.
*My favorite fans are Thermalright TY-14x series fans but there are many good fan, just use fans designed to work with airflow resistance.. with a good static pressure rating like those used on coolers. Reason is things like front vent and/or air filter, HDD cage, cables, etc. all create resistance similar to what a cooler does, so you need to overcome that resistant to get airflow. If there is not as much resistance it doesn't hurt anything... but have fans that cannot overcome the persistence means poor airflow.
*If you use PWM fans you can control them with motherboard CPU PWM and/or GPU PWM signal/s. To use motherboard CPU fan PWM signal all you need is a PWM splitter with molex power. Gelid make a good one. I don't like the Akasa braided ones because there are too many wires being made rigid than connecting to PWM plug and molex connector.. leads flex between connector and heatshrink on lead, stress fatigue and than start breaking off when you get in and out of as.. and fan either don't run or run 100% speed.
Using GPU PWM signal is more complicated because the GPU cover/cooler need to be removed to access the socket and than splice a lead to the PWM lead to supply a PWM splitter like above with PWM signal. If you do go that route I have drawings I can post. Controlling case fans this way (w/CPU &/or GPU PWM signal) means the airflow case fans supply increases and decreases the same time as CPU and GPU coolers need or don't need the extra air... so quiet when they are not working hard and only as loud as needed when working. I can only just hear mine at max load on system.
I know I've put up a lot of ideas. Feel free to use or not use as fits your needs.