An SSD will dramatically speed up anything that's I/O bound. Examples might include large compiles with lots of files, database work or slinging around large bodies of web content. Generally your punters' user experience on machines with SSDs will be much better.
Currently SSDs are quite expensive compared to consumer level hard drives or even enterprise class 15k RPM disks, but the cost of an X25M is still going to be a small fraction of the overall hardware and software costs of the desktop. If you're comparing it to an array of disks in the machine it may even be cheaper.
80GB is a lot of installed software and most development environments don't involve tens of gigabytes of data. For comparison, I have a development machine with VS, SQL Server, several business intelligence tools including MicroStrategy and quite a bit of other software, which takes about 45GB of disk space in total. The code base and other random files on the disk take about another 10GB(1).
You can manage disk space on an ad-hoc basis. If one of your punters runs out of space you can expand their machine by adding a second disk. SSDs might be prohibitively expensive if you're into HD video editing, genome sequencing or ETL development, but they're not likely to be a major issue otherwise.
NAS and gigabit ethernet are commodity items and make a good 'fast enough' platform for secondary storage unless you are working with large data volumes. Overall, I would expect workstations with SSDs and secondary storage on NAS hardware to perform well for most workloads.
For W7, the I/O latency is dominated by disk head seeks and rotational latency rather than bulk data transfer. A SSD will be dramatically faster on this type of workload than even a large number of disks in a RAID-0.
The SSD will be more reliable - no moving parts.
An SSD will run fine in a standard case/PSU.
For a Windows 7 developer workload I would expect to see SSDs provide by far the best performance. SSDs are already viewed as quite a good way to extend the usable life of a laptop - many models are sold with PATA interfaces for precisely this reason.
At a guess, I would say that a SSD would expand the lifespan of your computers quite substantially. Given the relative cost of hardware to salaries in Romania, that might be enough to make the business case by itself.
- They are somewhat pricey and not very large, but as above 80GB is actually quite a lot of room. Wear levelling is pretty much a solved problem in modern SSDs, so that's a non-issue these days.