Er, fair point, I managed to jump from "2gb is fine" to "2gb is what you have" via other posts. Dibbing in and out of a thread all day does not lead to a coherent train of thought.
2Gb is a minimum for Win7, make sure you're at that. 2gb DDR2 (Core2 processors and earlier) is about £15, so twice DDR3 (Core i processors and after). Anything Core i is likely to be on 2Gb anyway so I'd assume you need DDR2. It is still likely to be cheaper than replacing swathes of machines, though, although you can half and half - buy some new machines, gut the old ones for RAM.
Core2Duo \ 2GB RAM runs spot on here, slowest is E6600 and fastest E8400. SSDs will make a big difference but I wouldn't worry about a 3yr old CPU provided it's not an old Pentium \ Celeron \ AMD chip.
If your machines are pretty standard, either Dell \ HP \ Lenovo or Intel OEM desktop boards drivers won't be an issue either. Intel sometimes make you hunt for them (their support site reckons the DG965MS Stone board has no Windows 7 drivers yet if I download the chipset, VGA and sound drivers manually elsewhere on the site the whole thing runs perfectly!)
Try a few out first and see how they perform then make a decision based on if they're still under warranty and cost of upgrade vs replacement :)