Well, I just got my new computer which also happens to have an X550 512mb in it. Overkill, surely, but regardless having a new card has really made a difference in the image of my arcade monitor. Also, all the games seem different...faster, more articulate. It's almost like discovering MAME again.
The only reason I have this card is that the mobo was supposed to have integrated graphics - some recent or late model Intel 128mb deal - but they guy then realised that was the other board in the computer he'd built for someone else, so he got this card. I'd already ordered the avga, so am eagerly awaiting that, and may or may not return the X550.