When you say RAID, you need to specify the RAID type. Since you have an 80GB drive currently and are planning on only getting one more, I assume you mean RAID 0, which is striping.
The primary disadvantage of such a configuration is the loss in reliability. That is, if one drive fails, you lose both. Performance will probably not be MUCH better than if the drives were alone (and being used at the same time), but that depends mostly on the solution used to implement it. That is, performance won't improve by 100%, it will be more marked, probably more like 50% or less.
The caches are not combined; they are still specific to the drive they belong to.
So, in short, there are some pitfalls. But if you backup your data anyway, then why not?
