I recently returned from 2+ years volunteering in tropical Bolivia (see our blog here http://bo.teeks99.com/ ), doing a lot of what you are looking for. Right off the bat, you need to know it isn't going to be as easy as you think it will right now.
1. Shipping - The cost to ship a single 3-year old computer to a developing nation (don't forget import duties!) will probably be more than just buying a comperable computer in-country. Even though there aren't a lot of people with money in bolivia, there is still a great demand for computers. Thus the markets had lots of low-cost deals, not the latest technology but it was enough. Parts for a Core 2 Duo setup were ~$200, another $120 for a monitor. I would guess Guatemala would be similar.
2. Power - In every developing nation I've ever been in, electricity has been an issue for computers. In Bolivia it was rare that we would lose electricity completely (once every couple months), but very common where we would have brown-outs where there just isn't enough electricity in the wires. This is very bad for computers...so we had to hook every computer up to a battery (usually 2-3PCs for to a 1300VA battery). Unfortunately heat is the primary killer of batteries and the tropics is always hot. Over two years we probably spent 50% of the money spent on the lab on the original batteries and their replacements.
3. Administrator - Someone will need to be in-charge of the lab long-term. If you're there for a month you may have time to get it all setup and start training someone. However, it might not be that easy to get them up to speed on the basics of linux that they would need to know to keep it running. If there is just one big problem they can't solve they'll probably just go and install a pirated copy of windows on all the machines.
4. Cirriculum - I also wanted very much to teach the benefits of Open Souce, but that just wasn't what was needed by the students. The problem is that skills employers look for can be absurdly specific (probably due to the very different learning styles of students). It would not be uncommon for someone not to get a job because they had experience with Word 2003 instead of Word 2007. If you say..."I learned how to use Libre Office" the employer won't know what you're talking about, if you try to tell them it's basically the same as Microsoft Office they won't believe you. If you don't put "Learn to use Windows" or "Learn to use Office 2007" on the flyers for a course, you won't get very many students.
That said, I was able to make a decent multimedia course using Open Source: Gimp, Inkscape, Audacity, Kdenlive, etc., but to get students I also had to promise to teach them Photoshop, Illustrator, etc. What I ended up doing was speding a month with GIMP then just one day going through all the things they already learned in Photoshop...just so they could put it on their resume. (If you're interested, the text I wrote for my multimedia course is all open sourced too: http://mediaintro.sagradocorazon.info/ )
5. Teachers - Who is going to teach a cirriculum based on open source? There probably aren't already people there who can do it. Do you have a whole semester you could team-teach with one of the current teachers so that they would be equipped to continue on? What about when they get a different job, who will take over then?
I'm sorry if all this is a bit negative, I really do hope you have success. It would be amazingly helpful to the people in many developing countries if they had the tools of open source at their disposal (you could only imaging the countless hours wasted because of computer viruses that infect illegal copies of windows). I hope addressing these issues I had up front helps you save time in the long-run.