Processor speed, while important, probably isn't a factor in your problem. Anything over 1.5GHZ will play EB soundly.

It sounds more like a ram problem. If you're trying to play EB with 256 MB you're going to run into some serious lag. I would recommend updating to a gig, barring that, at least stick a 512 MB card in there to give yourself 768 MB.

You also might consider increasing your pagefile size...