Good for those projects when you need old fashioned peripherals, modest computing power, and a general purpose OS, but not one so intrusive that you can't get to the IO (i.e., WinNT+ -- though there are workarounds).
For those, I would recommend anything from an 8086 up to a Pentium, maybe Pentium 2-3 range. You'll have serial and parallel, ISA slots, and probably PCI on the newerish kind. Obviously, the older computers will be quite slow and limited in memory, but consider: 640kB of RAM is still competitive against modern budget DSOs in sheer storage (which have only relatively recently crossed into the "megapoints" range). Anything 486+ will be almost as fast, too (well, not counting any DSP or ASIC signal processing used to convert DAQ into FFTs and rasters..).
Even though the ISA bus and peripherals are hidden behind PCI bridges since the Northbridge-Southbridge system came out, the BIOS covers all that, so you still get transparent control over those devices.
Nice part about ISA being, you can use 74LS/HCT series chips directly on it, allocate your own IO range and talk directly with the system. PCI is possible too, but by then you're probably looking at an FPGA card to decode the gazillions of lines and handle PnP and all that.
Tim