Lysmerry Seconding Kyc's answer. I can't say how true to the Chinese text it is but I can't imagine a more fluid and lovely translation than the David Hawkes version.
Kyc The best translation would be that by David Hawkes (Vol. 1-3) and John Minford (Vol 4-5), available on Penguin Classics as The Story of the Stone. There is, however, a version with both Chinese and English text: ISBN: 9787119032184, where the translation is done by Gladys Yang and Yang Xianyi. However, I rate this translation as inferior to that by Hawkes and Minford.