I happened to need the equation for this while working up a new design for a low energy drift-tube RF accelerator (nothing so fancy as an RFQ, just the simple version). Strangely, it doesn't exist in any of my physics books as such -- I guess you're supposed to know Newton's laws, and wade through all the unit conversions forced on us by those who never want to have to write pi or c in equations (ESU, EMU, SI and so forth). Since nearly all modern texts ignore the specification of what units they use, and often mix them, there's no way to find out from the text alone -- you need a sanity check, a practical case, else you're likely to be off by pi or c (or some simple factor of those).
As usual, Fredrick Terman's Radio engineers handbook has it all when it counts for a practical engineer. I thought I'd put it here in case I lose that page again (these old WWII books fall apart on you and are getting scarce used).
For electrons:
V = 5.97 x 107 x sqrt(E) cm per sec. E in plain old volts. Mass contained in the constant.
For other charged particles:
V = 5.97 107 sqrt(EnMe/m) cm per sec. E in volts, Me mass of electron, M mass of actual ion, n number of unit charges on the ion
(this is page 275 in my copy, for reference) Of course, this is for non relativistic situations.
People wanting to mess with charged particles and magnets might also find this page scan interesting. Sorry 'bout the focus here, I have just one good copy of this and I'm not going to cut it up to lay flat. Once again, real units!