It's not likely to be the dongle's connection speed. YouTube videos are only around a couple Mbits/s - even the humblest of cards from a few years ago won't have difficulty with that throughput. I'd look at the ISP first, doubly so with Sky - does the problem only occur when wireless? (I've been exclusively wireless for several years, across a few machines and routers, though I first dipped my toe into the WiFi waters in 2001. That's not been the weak link)

Hold off spending that money!