We've installed solid state drives in all of our computers, recently, and today we installed a new gigabit router and a gigabit business class switch. Already opening ATX client list on a workstation is faster, and, of course, the SSDs make the computer faster than they were when they were brand new.
Next, we're going to rewire the entire office with latest Ethernet cable. The cable we have now is almost 20 years old and some of the ends are starting to crumble a bit.
The 500 GB SSDs were only 145 each, the router was 90 and the switch was 20 after a 10 dollar rebate. The wiring will probably be the biggest expense.