This phrase is in the description, I don't understand what it means.
So, say I have a DX10 card on Vista, are my specs too high? how does GoG intend on making it work? or should it just be fine and I'm being a worry wart?

You just need to make sure you have DirectX 9 installed.
Here is the latest:
August 2008
http://www.microsoft.com/downloads/details.aspx?FamilyID=2da43d38-db71-4c1b-bc6a-9b6652cd92a3&DisplayLang=en

So it is incompatible with DX 10?

If you have DX10, you can play any DX9 game. Vista does have some weird compatibility issues on some games though, but games from GOG.com will work on Vista and DX10.

DX10 and DX9 are two separate things. So technically, DX10 is not exactly a patch on DX9 so if you have DX10 you will have DX9 installed. Does that make sense? It's a bit confusing.

Yes it will be fine. However I would be more concerned over whether Second Sight will play nice with Vista. My guess is that it will, hence why GOG is adding it. I personally haven't tried it since installing Vista so I can't say for sure.

I was under the impression that DX10 is only backwards compatible to DX9?
I'm using Vista 64 and have installed DX9 also. DX9 is backwards compatible to DX6 as far as I am aware. This may help with games on GOG that use DX8 or even DX7.
Someone correct me if I am wrong.