About how long did it take you the first time you did it on one of the 06/08 models? Opening it doesn't seem that hard and I'm glad to know everything ends up working out better than you think when first starting it. I'm really stoked to do this. I was happy when I upgraded my MacBook's ram, but at that point the logic board hard already started failing and the Mac Mini will be able to use more of the ram. Do the screws go in and out easily everything go back together well? When I took the top casing off my MacBook I had a huge problem getting some of the screws back in and in fact, I couldn't even get four of them in.
Pretty much this;
I think the hardest part is getting the top case off. Once you manage to get that off, getting to the ram is pretty easy if you follow the ifixit guide. I'd guess around 30-40 minutes for your first time.
The tricky party is getting the blade into the gap and prising it correctly, especially if its your first time.
In terms of putting everything back together, it goes back perfectly. Once the top's off there are only about 4 screws and the threads don't seem particularly brittle. NB one screw is longer.
I would take care after disconnecting the airport antenna (check that the wire doesn't detach (unplug) itself from the board, and remember to disconnect the logic board wire at the front.
Also take care in flipping over the optical drive / HDD assembly (it sort of 'hinges' over still attached by a ribbon cable and things could get caught up). Provided you are careful it really shouldn't be a problem.
I recommend watching the OWC videos several times, and if need be have it running on your laptop / telly in the background, as you may be worrying about finding a screw that you dropped and you forget something else.
While you are in there, you may also want to consider replacing the HDD, as a larger capacity one isn't that expensive. Plus your intended mini is a little 'mature' now
One thing that might be worth bearing in mind though is that I added a 7200 rpm HDD to my current mini (2.53 Ghz C2D) and I don't know whether it was because of a crap drive, but the thing ran very hot, to the point where the fan was noisy constantly. I replaced this drive with a 1TB 5400 rpm jobby and whilst it possibly runs hotter than the original HDD it's no where near as bad as the 7200 rpm one.