It all depends on how you setup your server. If you allow everybody to chunkload as much as they want, it'll increase the need for server resources. If you let people create mystcraft worlds or if you don't pregenerate all the worlds, it might lag. If those 20 people are extreme players which build insane setups, it might lag.
It's not as simple as just putting a number on it. If there is lag, there is a reason for it. More often than people might think, it's not the RAM causing the issue.
One example might be flying around in the twilight forest with a power armor on. Generation of new chunks in the twilight is a laggy process. Do this with a power armor on flying really fast, you might give everybody lag.
If you pregenerate the twilight though, it won't need to generate any terrain when people fly around.
For the harddrive, will it cause lag? It might, it might not. A 500GB HD is very cryptic. You have quality classes in normal hard drives too. It also depends on wether you pregenerate your worlds or not. On how often you save the world to disk, and things like that.
