Coffee spilled inside tower, next steps

It's unlikely that the CPU was damaged unless it got really soaked. The coffee would have to somehow get underneath the heat spreader, or into the pins, both of which are covered up pretty well by the CPU heatsink and socket respectively. So if it was just getting sprayed around the case, probably not much happened to the processor.

The RAM is in the same boat - the important stuff is mostly covered up by the heatsink, which is just a plain piece of metal. So unless it completely drenched, you probably just have some crud caked on the outside and it will be perfectly normal if cleaned.

The hard drives are likely safe as well, since they are generally mounted in the front of the case and vent fans on the top are toward the back. So I doubt a lot of liquid made it all the way there.

GPU, PSU and motherboard are most likely to be ruined. Although since it sounds like the machine was off when this happened, you may have lucked out. Where you get really bad damage is when the machine is on and something gets spilled in it; then it can cause a short which can fry just about any component in the system. If it got wet while powered off, the damage could be much less or even nothing if you are lucky.

What I would do is try to borrow a different PSU, remove the graphics card, and see if you can power on the system with onboard graphics if you have them, which should work unless the motherboard is wrecked. Then find an old junker system to test out the two most suspect components, the PSU and GPU. First just swapping the PSU, then adding the GPU.

If there is a second PCIe x16 slot, you may want to use that from now on, assuming the system works. Although really the main thing you are worried about there is that the contacts are clean. If you can accomplish that, and the motherboard itself is intact, then not too many worries. A good thing to use for that are the cleaning cards you can get for some printers and scanners - basically a thin card with cotton on it that you apply alcohol to. They're designed to clean the rollers on those devices and fit through a thin track about the width of a piece of paper, so it's a close enough fit a lot of the time.

edit: wait, no, it looks like the power may have been on. So you'd be really, really lucky if the GPU is intact, and pretty lucky for the power supply. The rest I still have a good feeling about.

One more lesson: Don't use a case with a top-mounted fan unless it is UNDER the desk, or WELL away from it. I can't tell you how many times I've heard of anyone from the user himself to the kids to the cat knocking over a cup of liquid into the top vent, although it's usually soda. It is not a rare occurrence.