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Do you think that the program would run faster with 4Gb or SSDs? I don't care how long it takes to load, only to complete events.

SSDs most likely. XP usually does just fine with 2 or 3GB. If you have Win 7/Vista, 4GB seems to be that new sweet spot. However, most XP computers have old hardware that can't take advantage of the new SATA 6G/SATA3 standard. To achieve the speeds you desire, my guess is that you'll also need SSDs on the server. With SSDs on clients and hard drive on server, I'm seeing about 1-2 seconds. For SSDs on both server and client, <1 second. Here is a video I recorded from a remote session (3 doc practice as well) so it might even be faster in person. Lots of variables to consider when doing remote work. Should be obvious that it's running on a client computer as well with the Essentials launchpad in the video.

Allergy Demo over RDP
Allergy Demo over RDP (HD 720P)
Amazing Charts Official Allergy Demo Speed <- Is this your current speed?

Personally, I think 2 seconds is acceptable, but it's up to you in the end. Not sure how much an extra second is worth to you. Also, to be honest, I don't think it ranks up very high for AC developers with the big 81 and new MU requirements coming.

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I'm hesitant to virtualize the network. Might do that when it's time to update the clients, or maybe just for the exam rooms.

I don't think I mentioned virtualizing. But you could make a terminal server that is dedicated to running AC instead of upgrading each computer. (depending on how many you have) If it runs that quickly for me over the Internet, I'm sure it'll be even more pleasant experience over LAN.