1) It would be 'best', yes. But not a true requirement.
DDR400 indicates the type (
DDR SDRAM DIMM) and speed (400Mhz) of the memory. If you put in slower memory then your memory will run at the speed of the slowest stick. However, besides slower speed, you will have no troubles with that.
2) Check your computer manual for this! Not all combinations of sizes are supported by all brands/models of PCs.
(But, it doesn't have anything todo with memory speed though, wolfeboy100)
3) Kingston is one of the most spread memory brands. They provide an extremely good price/quality. In fact, they have a lifetime-long garantuee on all their memory modules. If it breaks or fails (and it isn't your fault of course) they provide you with a free replacement.
You can buy cheaper modules from other brands (as long as you buy the correct memory type of course), but you wont have the same garantuee (and quality), or you can buy much more expensive ones, but all in all, Kingston is never a bad choice.
/ 
