Even for basic administrative tasks, it has repeatedly been shown by lots of studies over recent years, that computer users with two or even three monitors are more productive than users with a single monitor.
A dual or triple monitor setup has been "standard fit" for software developers and web designers for many years, and now with the price of large monitors at a low, there's no reason why your normal desktop office users can't benefit too. If you think about it, it does make sense. The productivity increases are most noticeable for tasks that require reference to another program or document, for example working on a big Excel spreadsheet whilst looking at figures in another. On a single monitor, you have to keep switching between applications, on a dual monitor setup, you can work on one monitor, and refer to another document on the other.
Also, it's been shown that when users switch to another task temporarily, eg. to look something up online, or check email, because the original task is now no longer visible, the user tends to forget about it, and focus on the distraction!
It's also a bonus to keep your regular applications visible to you. With two screens, you can keep your "reference" applications on one screen, things like your email client and your contact manager, and use the main screen for whatever task you are working on, eg. Excel, Word etc...
A second 22" monitor can be had for as little as £110 + VAT at the time of writing (late 2012), and many desktops PC's will support dual monitors out of the box. If not you will need to factor in a basic graphics card to give you a connection for the second display, typically £50 or thereabouts. So for an extra £160 or so, you can see significant productivity boosts, particularly in your heavy computer users.
Don't re-use you old monitors!
If you're spending money on replacing PCs in your business, we'd really encourage you to replace the monitors too. Fair enough if your budget is really tight, but it's a good opportunity to keep your staff happy and increase productivity. If the machines you're replacing are a few years old, the monitors are probably only 17" in size. Standard monitor size for a new office PC is arguably 22" or even 24", as the price is comparable with what you paid for the 17" monitor when you bought it! Those extra inches make all the difference, and if you can stretch to it, why not go for two displays?