There are a few parts to your question, and the last few words really compelled me to answer. I've been "retro-gaming" on Slackware for a couple of decades... and let me tell you that it has never been as good as it is now.
Part 1 - Hardware
Any GPU should suffice to play 20 year old Windows games. My main desktop machine doesn't have a discrete GPU... I use the on board graphics to play Warcraft 3 with all settings at the maximum possible. I haven't tried WoW but it's from roughly the same era, so I'd be surprised if it doesn't run.
Part 2 - Software
In terms of the software that is needed to run old Windows games, there are 2 "layers" which you need to add to Slackware to make this work:
Multi-lib
Still to this day, a significant amount of Windows software is 32 bit. Part of the reason that Windows is so big is that it has it's own proprietary and extremely well integrated multi-lib system. It's so good, and so tightly integrated that you don't even know it's there. There is no such thing as pure 64 bit Windows. What does this mean for you, a Slackware user? Essentially, you need a set of 32 bit libraries on your machine to run 32 bit software. A stock installation of Slackware64 has none of these libraries by default.
Wine
Wine is essentially a Windows API compatibility layer, which will allow Windows binaries to run under Linux. It has been in development for a very long time and is quite mature and capable.
How do you get both of these?
There are (at least) a couple of answers to that question, but the end results are the same regardless of the path you choose. The main question you need to ask yourself is: Do you want to spend time setting up (and maintaining) your system? Or would you rather just jump in and start playing?
I'm a big fan of doing things the easy way... Particularly if there is no perceptible benefit to doing things the "hard" way. It doesn't get much easier than this:
https://github.com/Kron4ek/Conty/releases/tag/1.20 Conty.sh is a Linux container. Think of it as a huge static binary which contains all of the required libraries and binaries with a few extras thrown in as well. It's a big download, but once it's downloaded you can use it straight away. The best part is that it runs unprivileged... It can't make any changes to your system outside of your user account.
Running it is a matter of calling it like this, for example to run Steam (which is built in):
Code:
~$ ./conty.sh steam
Or to use Warcraft under Wine:
Code:
~$ cd /path/to/warcraft
$ /home/user/conty.sh wine war3.exe
I could write about gaming in Slackware all day, but I'll stop there. Enjoy!