I posted a similar query a while ago and got a mix of responses. DirectX is (simply) a common set of graphics and sound instructions that enable your games to address your system and run. It is vital for most modern games. The concensus of opinion when I asked the question (albeit under my Win98 system) was that if you're on DX8.1b or thereabouts, then there appears little benefit in upgrading to DX9 ..... at the moment. At the same time there were some that had upgraded and had experienced problems. In short, if it's working OK at the moment and you're reasonably up to date, then leave well alone for the time being.