If you don't itemize your deductions, this is a moot issue. If you do itemize, read on.
You need to value them at their fair market value. That is, what is the going price? I'd assume that they are roughly $100 each, but you should check ebay selling prices or used computer stores in your local area.
If you donate less than $500 worth of stuff, it is simply based upon your word. You really don't need any proof of value. If you donate more than $5000, then you need appraisals which may be costly or just a hassle (and you have to fill out the lengthly version of IRS form 8283).
I assume your donation is in the ~$2000 range. Thus, you are in between the two cases in the paragraph above. You'd have to fill out form 8283, but you could leave almost all of it blank. Then we get to what I think is a grey area. If you donate more than $500 worth of an "item", you need to give a detailed method of how you determined the fair market value. Are a bundle of 20 computers an "item" or are they 20 items? I'm not a tax expert, so I don't know. But, if you donate 4 bundles of 5 computers with each bundle worth ~$400, then you know for certain that no donation is worth more than $500. In which case you don't even have to specify how you valued the computers (but you should at least have a rough estimate by checking ebay or similar, and print a page with the ebay prices on the date of the donation).
Long winded conclusion: donating in smaller bundles of ~4 computers to several charities may make this a lot easier when tax season comes.