It should be ok, although you will likely lose some speed with an ethernet cable that long, which could lead to some buffering while streaming video.
One option is to use a homeplug solution. This article, Options for connecting your Slingbox to a router in a different room demonstrates this option.
Hope this helps,
If I remember correctly, the spec for cat5 ethernet is maximum single hop of 100 metres.
But I know from experience (in industrial environments) that this is not normally easy to achieve.
It requires best quality copper cable (not the cheaper copper coated steel), also good quality terminal devices (router etc).
But usually 50m or so is quite ok and works fine.