OptiPlex 745 Ram sticks

SpeedStep is absolutely right and that's why memory vendors tell you to buy RAM in pairs for an Optiplex 745.

However, in your case you could just buy 1 "stick" IF YOU BUY THE EXACT SAME DIMM (stick) AS THE ONE THAT DIED.

It is also sometimes possible to mix 2 RAM DIMMs ("sticks") that are almost the same, but just slightly different, in the same channel (white or black slots) and get away with it. I do not recommend that for you and it is something that I generally stay away.

So, here are your options:
1 - stay with your 3 working DIMMs = you have less RAM
2 - get the exact same DIMM as the one that died = same RAM as before
3 - get a pair of DIMMs either new or used = maybe change, maybe not

You haven't told us if you have a 745 minitower, desktop or SFF. You can't have a USFF because that only has 2 DIMM slots. You also haven't told us what the DIMM size is that you have in your machine. I guess it might be 2 GB, 1 GB or maybe 512 MB.

Your machine is a bit old and you might want to consider upgrading to a more recent model. It doesn't have to be a new machine. You could get a used Optiplex 790 or whatever ... just something more recent.

If you are going to stay with the 745 and you had a 1 GB DIMM die on you then you might possibly upgrade that channel to 2 GB DIMMs. That means that you buy 2 DIMMs and you would have an extra (useless) 1 GB DIMM on you hands after the upgrade. I'm hoping that you don't have 512 MB DIMMs.

You have options ...

Option 1 of staying with 3 DIMMs may not be such a good one. Typically, a dual channel motherboard like yours will only be happy with an odd number of DIMMs if there is just 1 DIMM plugged into the board. 3 DIMMs in that kinda mobo can easily stop the mobo from booting. If yours boots with 3 then it is at least partially OK with that config and might be fully OK with it. I guess that it might be possible for it to boot and yet still ignore the 3rd DIMM. You would have to check with OS.

Option 2 of replacing the dead DIMM with another of the [exact] same is pretty good. You can very likely get another used DIMM on either eBay or Amazon. Both have good guarantees. If you receive a non-working DIMM (rather unlikely) you either get your money back or you get another DIMM, but you have to complain to the seller/eBay/Amazon in order to get the replacement DIMM. It may or may not be difficult to find a brand new exact replacement for your dead DIMM. It depends on the DIMM.

Option 3 is pretty much like option 2, except that you might be able to upgrade to 2 GB DIMMs if that is not what you have now. Both new and used DIMMs are options here.

Your 745 takes 1 GB and 2 GB DDR2 DIMMs. Those are pretty cheap these days if you buy them used. In my experience used RAM from eBay/Amazon is just as good as new, but YMMV. Some folks just feel better about buying new parts and getting a 1 yr/90 day/whatever guarantee. Choose wisely, grasshopper.