You probably have a diagonal screen size, and a resolution.  Use the Pythagorean Theorem on the resolution and divide the result by the screen size to get the DPI.

For example, my Vibrant is 4", 800x480.  sqrt(8002 + 4802) / 4 = ~233 dpi.

What is my lcd density?
If I want to run a few dev VMs on a laptop, how fast do my drives realistically need to be?  I do not expect lightning fast performance, but the last time I tried this on a Lenovo Thinkpad T400, suspending to disk takes forever, and the host OS (Windows 7 x64 Professional) grinds to a halt with VMWare Workstation 6.5.  I understand this is going to happen, and this is not the right platform in general, but how much better would I have it with a 7200 RPM drive?  Are there faster drives available?  I really want to have the flexibility of VMs on the go, and it takes 3+ or 4+ minutes the last time I tried.  The current one is really crappy.  I am shopping for a new computer anyway, and I was hoping someone could tell me this is a worthwhile consideration for now and the future or just a pipedream.
