That mobo takes a socket 754 AMD processor, which is an obsolete design. The processors are still available but no longer being manufactured. For this reason your best deal is almost certainly to buy the bundled processor with the mobo. Add RAM, Hard drive, DVD-RW drive, and a case, and you've got the whole package.
It uses standard DDR RAM (Not the newer DDR2) but that is still widely available so no worries there. Crucial and Kingston brands have the best reputation. I suggest buying a gig as it's pretty cheap. The board has a SATA connection so you can use either type of hard drive (SATA or PATA/EIDE). I prefer Seagate because of the longer warranty.
Being an older design you won't get blazing performance from this combo but if you're just looking for a machine to do things other than gaming this will be fine.
If you want to re-use your old case case make sure the power supply is up to the job. Suggested minimums from AMD used to be +3.3V 28 amps. +5V 30 amps. +12V 15 amps. Don't pay too much attention to watt numbers, those can be misleading. Since modern motherboards power the processor from the 12 volt line, I'd suggest going higher than the minimum on that one. Also make sure the power supply has the little 12V 4-pin plug (a separate connection for the processor) as well as the standard ATX connector.
Frankly if your case is from the Windows 98*Grinler era you'll probably need a new PSU (power supply unit). It would be worth your while to visit a store that sells them just to get a look at how they attach to the case. If your case is generic a newer PSU will probably fit. If you have a brand-name PC (e.g. Compaq) you may have to make some modifications, or else buy a new case.
If you have specific questions about the Sempron 64 and compatible motherboards, RAM, power supplies, etc. this forum
is the place to go. Don't be put off by the high level of some of the talk, they are happy to answer basic questions. I used to hang out with some of the guys there on another (now-defunct) forum.