Cooling Servers in Unfinished Basement?

How high is the ambient temperature? Maybe it's just fine the way it is? How hot is the CPU? Speedfan software can help identify this. That's more of the "bottom line" with respect to do you really need to do anything. That factors in ambient, CPU cooling and air distribution, load you are putting on it, etc.

What geographic area? ie: north / south?

You might be able just to put a standard fan on to better distribute the current ambient air to make the PC less of a hot spot.

Is the PC in a rack?

Try speedfan and it will give you all those info you are looking for.

I've a home automation PC and media server in my garage and they are running about 45C for CPU, 40C for internal temp and 47C for the HDDs.

100F degrees is about 37C ... your room is plenty cool then. It may feel warm to us, but computers are designed to withstand high temps. If you have ever walked through an actual server room, with out the extreme cooling some corporations have now, you would be amazed at how warm those rooms are. Ours at work is always on the "hot" side. It wasn't designed to handle the amount of computers we now have in there 10-15 years ago when it was built, yet we have never had an issue related to heat.... except for a few of us who complain about it while changing out a tape/backup drive or swapping cables... haha.

Anything below 70C is safe, and anything below 50C is excellent. I don't think you have anything to worry about with 37C CPU temps.

According to Intel, PCs designed for home/office generally have a maximum rated ambient temperature of 35 degrees C.

That's ambient (room temp) and not CPU temp. So basically, they are designed to operate in rooms up to 95 F. Most basements are MUCH cooler than that.