What I vaguely remember is that Xander is driving his car. Tara is with him and is showing him a ring that she bought for Willow. There's a car accident (Xander's fault, I think) and Tara is killed. He and Willow slip into a quasi-relationship; out of Xander's guilt I think and Spike snaps them out of it.
I know, not much to go on...but it is driving me crazy...crazier...
Found: Thank you, thank you, thank you!